OK, so let's see how this would work out.
Let's suppose a US judge agreed that it was a survey error and that that land had to be returned to Canada. Think of the mess this would create. Any person born on Canadian soil is automatically a Canadian citizen. This means that since a judge would have ruled it a survey error, that land would have always been Canadian for over the last century, and so any person born on that land could now claim Canadian citizenship. Automatic dual citizens right there. I suppose the US could revoke their US citizenship, but for compassionate and other grounds, they'd probably get to keep their US citizenship. That would then raise the question of whether any person born on that land could run for President. The US would need a Constitutional amendment to fix that if it wanted to do that.
Since the land would always have been Canadian, Canada could retroactively claim all taxes collected on that land for the last century and more while the US could claim compensation for all the public funds spent on that land for all of that time. I presume that both sides would call a truce on that one and just start counting from now on.
Meanwhile, Canada would have to take over US istitutions in the area and adapt them all to Canadian laws. Just confiscating firearms would take forever, not to mention the moaning and bitching we'd have to put up with from the locals over all of the changes. And then we'd have to provide bilingual services in French and English too. Just imagine the moaning and groaning from aspiring public servants who'd spent years studying English and Spanish in high school only to now learn that they should have learnt English and French instead!
Sorry, but it's just not worth the pain in the rear end. If a US judge ever declared that the US had to hand that land over to Canada, Canada should present a treaty instead that would just retroactivaly make it a US territory since the beginning but add an open border from now on comparable to Svalbard (i.e. with open borders that would allow any Canadian to visit, study, work, or do business there visa-free as long as he supports himself and respects local laws).
Since the new treaty would retroactively make it US territory, this would ensure that the locals would have always been born on US territory and so could still run for president if they wanted to. Meanwhile, Canada would save itself the trouble of having to inherit a bunch of unwilling new citizens and needing to hand them all new birth certificates identifying their country of birth as Canada. Problem solved!