None of this matters. You’re a Polish citizen, you have a right to be in Spain more-or-less indefinitely (with some small caveats but nothing that matters in this scenario). Using your Canadian passport doesn’t change that basic fact and, in the highly unlikely event that someone anywhere in the EU ever questions you about that stamp or any incomplete travel record, establishing your Polish citizenship will be enough to clear up the issue.
I crossed back in Spain from Gibraltar on my Polish passport, no issue—they didn’t scan it or stamp it or check or anything, just waved me through.
That’s pretty typical. Spanish border guards are aware of the rules I just mentioned. They are forbidden from stamping your Polish passport and have absolutely no reason to check anything else. They could scan it if they suspect it’s fake or stolen or want to check if there is an alert for your arrest but other than that they would not deny you entry. In particular, previous stays or the purpose of your travel are completely irrelevant.
I am not so sure about the Gibraltar side of things. The UK relies on passenger manifests and data from transportation companies but with Gibraltar being accessible by road and still figuring out its relationship with Spain after Brexit, I am not up to date on exactly how they handle immigration.
Importantly, overstaying is a material fact. If you are out of the territory, you cannot be overstaying. What a stray stamp could potentially do is create some confusion or reverse the burden of proof but it doesn’t mean you’re overstaying anywhere. Again, I don’t know about Gibraltar but EU law is explicit about that.
Generally speaking, oversights are not uncommon and there are a lot of inconsistencies in stamping. For the UK specifically, you can always mention your Polish/EU citizenship and show your other passport while still using the Canadian passport.
You are overreacting, as you guessed.
First of all, if there is any question at any time, you can show both passports and explain that you expected to get an entry stamp into the UK/Gibraltar. Neither Canada nor Poland has any problem with dual nationality.
But even more important, that stamp in your passport is just that, a stamp, it does not have a corresponding digital registration as those are not yet made in the Schengen area. This makes it very unlikely you will ever be asked about that exit stamp.
Other countries may be interested if you apply for a visa, but that should not be a problem either, as you can proof you entered the Schengen area on your Schengen area passport.
Credit:stackoverflow.com‘