Did the Buddha ever say something proven wrong later?

I think Buddha himself has answered this question in Maha-sihanada Sutta,

23. Here, I see no ground on which any recluse or brahman or god or Mara or Brahma or anyone at all in the world could, in accordance with the Dhamma, accuse me thus: 'While you claim full enlightenment, you are not fully enlightened in regard to certain things.' [72] And seeing no ground for that, I abide in safety, fearlessness and intrepidity.

24. "I see no ground on which any recluse... or anyone at all could accuse me thus: 'While you claim to have destroyed the taints, these taints are undestroyed by you.' And seeing no ground for that, I abide in safety, fearlessness and intrepidity.

25. "I see no ground on which any recluse... or anyone at all could accuse me thus: 'Those things called obstructions by you are not able to obstruct one who engages in them.' And seeing no ground for that, I abide in safety, fearlessness and intrepidity.

26. "I see no ground on which any recluse... or anyone at all could accuse me thus: 'When you teach the Dhamma to someone, it does not lead him when he practices it to the complete destruction of suffering.' And seeing no ground for that, I abide in safety, fearlessness and intrepidity

MN 12

While this statement points to the omniscience of Buddha, it also shows his confidence that no thing in this world is able to challenge him and his teachings.

I would probably say that Goatama's medicine was infallible, but certainly would say that he was not omniscient in the Western sense.

I have read a zen master say he wasn't, too (IIRC!).

But, perhaps he stayed silent (like Wittgenstein suggested) about what he didn't know, and he knew what he didn't knew.

IMVHO a better potentially more fruitful way of thinking about it would be to ask: did Goatama know everything there was to know about his Buddha field? And then, what was his Buddha field, did it include everything that happened or only the way sentient beings were suffering.

Yes, there are such things. For instance, the Buddha taught that the universe consists of four basic elements (mahābhūta): earth, water, fire and air. Now we know with reasonable certainity that they are not basic elements in any sense: fire is a chemical process, water is a chemical compound, air is a mix of gases and earth contains several chemical substances.

Aṅguttara Nikāya

The Book of the Fours 24 Kāḷaka

Thus have I heard. On one occasion the Blessed One was dwelling at Sāketa, at Kāḷaka’s Park. There the Blessed One addressed the bhikkhus: “Bhikkhus!”

“Venerable sir!” those bhikkhus replied. The Blessed One said this:

“Bhikkhus, in this world with its devas, Māra, and Brahmā, among this population with its ascetics and brahmins, its devas and humans, whatever is seen, heard, sensed, cognized, reached, sought after, examined by the mind—that I know.

“Bhikkhus, in this world with its devas, Māra, and Brahmā, among this population with its ascetics and brahmins, its devas and humans, whatever is seen, heard, sensed, cognized, reached, sought after, examined by the mind—that I have directly known. It has been known by the Tathāgata, but the Tathāgata did not become subservient to it.

“Bhikkhus, if I were to say, ‘In this world with its devas … whatever is seen, heard, sensed, cognized, reached, sought after, examined by the mind—that I do not know,’ that would be a falsehood on my part.

“Bhikkhus, if I were to say, ‘In this world with its devas … whatever is seen, heard, sensed, cognized, reached, sought after, examined by the mind—that I both know and do not know,’ that too would be just the same.

“Bhikkhus, if I were to say, ‘In this world with its devas … whatever is seen, heard, sensed, cognized, reached, sought after, examined by the mind—that I neither know nor do not know,’ that would be a fault on my part.

(1) “So, having seen what can be seen, the Tathāgata does not misconceive the seen, does not misconceive the unseen, does not misconceive what can be seen, does not misconceive one who sees. (2) Having heard what can be heard, he does not misconceive the heard, does not misconceive the unheard, does not misconceive what can be heard, does not misconceive one who hears. (3) Having sensed what can be sensed, he does not misconceive the sensed, does not misconceive the unsensed, does not misconceive what can be sensed, does not misconceive one who senses. (4) Having cognized what can be cognized, he does not misconceive the cognized, does not misconceive the uncognized, does not misconceive what can be cognized, does not misconceive one who cognizes.

“Thus, bhikkhus, being ever stable among things seen, heard, sensed, and cognized, the Tathāgata is a stable one. And, I say, there is no stable one more excellent or sublime than that stable one.”

Amid those who are self-constrained, the Stable One
would not posit as categorically true or false
anything seen, heard, or sensed,
clung to and considered truth by others.

Since they have already seen this dart
to which people cling and adhere,

saying “I know, I see, it is just so,”
the Tathāgatas cling to nothing.

The problem I have faced with your question is the only teaching of the Buddha of which I am totally convinced is genuine is the flower sermon.

There was nothing said. Obviously Shakyamuni Buddha gave sermons, but what was in those sermons will be debated by theologians. To me what has been said that we can verify with our practice is the basis of human suffering and the way out of it. The principles we can verify with our practice. If we did find something he reportedly said that was wrong or false, how would we know he really said it? This is the downfall of trying to do a meticulous study of an oral tradition over 2000 years old. It does not invalidate Buddhism's usefulness but does preclude in my opinion any proving of points of details about what the Buddha said.

The Note at the bottom of this Sutta says,

This translation follows the Thai and Burmese versions of this passage. The Sri Lankan version replaces Ven. Nanda in this list with Ven. Ānanda; the PTS version replaces him with Ven. Devadatta and Ven. Ānanda. These latter two readings would appear to be mistaken, as the Buddha in this sutta defines "brahman" as one whose fetters are ended — i.e., an arahant — whereas Ven. Ānanda became an arahant only after the Buddha's passing; Devadatta, after having caused a split in the Saṅgha toward the end of the Buddha's life, fell into hell.

So according to one version (i.e. the Pali), he praised Devadatta, and may therefore have been "proven wrong" by Devadatta's subsequent misbehaviours.

This is a famous dilemma, given that Devadatta has a reputation as an evil-doer (who tried to kill the Buddha, and cause a schism within the order): "Perhaps the Buddha shouldn't have admitted Devadatta to the Sangha."

The Questions of King Milinda include the following question and answer,

'What then, Nâgasena! Was the Buddha aware that Devadatta after being admitted to the Order would raise up a schism, and having done so would suffer torment in purgatory for a Kalpa?'

'Yes, the Tathâgata, knew that.'

'But, Nâgasena, if that be so, then the statement that the Buddha was kind and pitiful, that he sought after the good of others, that he was the remover of that which works harm, the provider of that which works well to all beings--that statement must be wrong. If it be not so--if he knew not that Devadatta after he had been admitted to the Order would stir up a schism--then he cannot have been omniscient. This other double-pointed dilemma is put to you. Unravel this tough skein, break up the argument of the adversaries. In future times it will be hard to find Bhikkhus like to you in wisdom. Herein then show your skill!'

'The Blessed One, O king, was both full of mercy and had all knowledge. It was when the Blessed One in his mercy and wisdom considered the life history of Devadatta that he perceived how, having heaped up Karma on Karma, he would pass for an endless series of Kalpas from torment to torment, and from perdition to perdition. And the Blessed One knew also that the infinite Karma of that man would, because he had entered the Order, become finite, and the sorrow caused by the previous Karma would also therefore become limited. [109] But that if that foolish person were not to enter the Order then he would continue to heap up Karma which would endure for a Kalpa. And it was because he knew that that, in his mercy, he admitted him to the Order.'

[etc.]

Devadatta is also mentioned as follows in the Mahayana Lotus Sutra,

I announce to you, monks, I declare to you: This Devadatta, the monk, shall in an age to come, after immense, innumerable Æons, become a Tathâgata named Devarâga (i. e. King of the gods), an Arhat, &c., in the world Devasopâna (i. e. Stairs of the gods).

The Buddha's Dhamma is Universal Law of Nature, open for scientific examination, timeless, when practiced lets you to understand the ultimate realities and is something that can be realised at the experiential level.

There can be issues with the different interpretations after 2500+ years and some may be miss specified in transmission, and some sayings may be even later compositions incorrectly attributed to the Buddha.

But having said this there cannot be anything which the Buddha said which can be wrong since what he taught is (according to him) what he experienced directly which can also be experienced by any body by practicing the well laid out path.

Ultimately what you should believe is what you experience for your self within the framework of the body. This is what you should take as the teaching of the Buddha, as he has emphasised.

The 6 qualities of the Dhamma in detail:

Svākkhāto (Sanskrit: Svākhyāta "well proclaimed" or "self-announced"). The Buddha's teaching is not a speculative philosophy but an exposition of the Universal Law of Nature based on a causal analysis of natural phenomena. It is taught, therefore, as a science[11] rather than a sectarian belief system. Full comprehension (enlightenment) of the teaching may take varying lengths of time but Buddhists traditionally say that the course of study is 'excellent in the beginning (sīla – Sanskrit śīla – moral principles), excellent in the middle (samādhi – concentration) and excellent in the end' (paññā - Sanskrit prajñā . . . Wisdom).

Sandiṭṭhiko (Sanskrit: Sāṃdṛṣṭika "able to be examined"). The Dharma is open to scientific and other types of scrutiny and is not based on faith.[12] It can be tested by personal practice and one who follows it will see the result for oneself by means of one's own experience. Sandiṭṭhiko comes from the word sandiṭṭhika which means visible in this world and is derived from the word sandiṭṭhi-. Since Dhamma is visible, it can be "seen": known and be experienced within one's life.

Akāliko (Sanskrit: Akālika "timeless, immediate"). The Dhamma is able to bestow timeless and immediate results here and now. There is no need to wait for the future or a next existence. The dhamma does not change over time and it is not relative to time. Ehipassiko (Sanskrit: Ehipaśyika "which you can come and see" — from the phrase ehi, paśya "come, see!"). The Dhamma invites all beings to put it to the test and come see for themselves.

Opanayiko (Sanskrit: Avapraṇayika "leading one close to"). Followed as a part of one's life the dhamma leads one to liberation. In the "Vishuddhimagga" this is also referred to as "Upanayanam." Opanayiko means "to be brought inside oneself". This can be understood with an analogy as follows. If one says a ripe mango tastes delicious, and if several people listen and come to believe it, they would imagine the taste of the mango according to their previous experiences of other delicious mangoes. Yet, they will still not really know exactly how this mango tastes. Also, if there is a person who has never tasted a ripe mango before, that person has no way of knowing exactly for himself how it tastes. So, the only way to know the exact taste is to experience it. In the same way, dhamma is said to be Opanayiko which means that a person needs to experience it within to see exactly what it is.

Paccattaṃ veditabbo viññūhi (Sanskrit: Pratyātmaṃ veditavyo vijñaiḥ "To be meant to perceive directly"). The Dhamma can be perfectly realized only by the noble disciples (Buddha) who have matured in supreme wisdom. No one can "enlighten" another person. Each intelligent person has to attain and experience for themselves. As an analogy, no one can simply make another know how to swim. Each person individually has to learn how to swim. In the same way, dhamma cannot be transferred or bestowed upon someone. Each one has to know for themselves.

Sourced: http://en.wikipedia.org/wiki/Dharma_(Buddhism)