Not necessarily a "tech" oops moment, but many years ago, I accidentally boarded a wrong flight and realized after fighting for the same seat with another passenger for few mins. Luckily, I was able to get out on time and run to my real flight. This is how the conversation went:

Me: Sir, this is my seat.

Person: Are you sure ? I think this seat is mine.

Me (annoyed but confident): Please check your boarding pass

Person: Ok let me check

...In the meantime, someone yells from another seat "You guys both going to austin/texas right?"

That was my oops moment. I was not going to texas of course. That seat was half way through the plane and people were still getting in. :)