Unintentional karma and its effect

"Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect.
Nibbedhika Sutta

Also from Cetana Sutta (SN 12.38) below, we see that intention (cetana) provides a landing point for the continuation of karmic consciousness.

[the Blessed One said,] "What one intends, what one arranges, and what one obsesses about: This is a support for the stationing of consciousness. There being a support, there is a landing [or: an establishing] of consciousness. When that consciousness lands and grows, there is the production of renewed becoming in the future. When there is the production of renewed becoming in the future, there is future birth, aging & death, sorrow, lamentation, pain, distress, & despair. Such is the origination of this entire mass of suffering & stress.
Cetana Sutta (SN 12.38)

The story of blind elder monk Cakkhupala from Dhammapada 1 shows the importance of intention. It also shows that unintended action has no consequence.

On one occasion, Thera Cakkhupala came to pay homage to the Buddha at the Jetavana monastery. One night, while pacing up and down in meditation, the thera accidentally stepped on some insects. In the morning, some bhikkhus visiting the thera found the dead insects. They thought ill of the thera and reported the matter to the Buddha. The Buddha asked them whether they had seen the thera killing the insects. When they answered in the negative, the Buddha said, "Just as you had not seen him killing, so also he had not seen those living insects. Besides, as the thera had already attained arahatship he could have no intention of killing and so was quite innocent."

Verse 1: All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' follows him just as the wheel follows the hoofprint of the ox that draws the cart.
Dhammapada 1

Also the story of the hunter's wife from Dhammapada 124 shows the importance of intention. It also shows that unintended action has no consequence.

The bhikkhus then asked the Buddha, "Venerable Sir, is the wife of the hunter who is a sotapanna, also not guilty of taking life, if she has been getting things like nets, bows and arrows for her husband when he goes out hunting?" To this question the Buddha answered, "Bhikkhus, the sotapannas do not kill, they do not wish others to get killed. The wife of the hunter was only obeying her husband in getting things for him. Just as the hand that has no wound is not affected by poison, so also, because she has no intention to do evil she is not doing any evil."

Verse 124: If there is no wound on the hand, one may handle poison; poison does not affect one who has no wound; there can be no evil for one who has no evil intention.
Dhammapada 124

So, intention is kamma. You can see in this question that buying frozen meat from the supermarket does not incur the kamma of killing, because there was no intention or action to kill.

But does this mean that intention without acting upon the intention can result in kamma?

When a warrior strives & exerts himself in battle, his mind is already seized, debased, & misdirected by the thought: 'May these beings be struck down or slaughtered or annihilated or destroyed. May they not exist.' If others then strike him down & slay him while he is thus striving & exerting himself in battle, then with the breakup of the body, after death, he is reborn in the hell called the realm of those slain in battle.
Yodhajiva Sutta

Here, the warrior has the intention. But when he strives and exerts himself based on the intention, then the fruit of the kamma reaches its peak.

Despite this, intentional thoughts are also intentional kamma that can result in suffering.

After doing an intentional kamma by way of body, speech and mind (whose result is) to be felt as pleasure, he feels pleasure; after doing an intentional kamma by way of body, speech and mind (whose result is) to be felt as pain, he feels pain; after doing an intentional kamma by way of body, speech and mind (whose result is) to be felt as neither-pain-nor-pleasure, he feels neither-pain-nor-pleasure.
Mahakammavibhanga Sutta