The only time I boarded the wrong flight was at an airport where the JetWay was a "Y" configuration leading to 2 separate planes. Which they happened to be boarding 2 at the same time!

I didn't know they were boarding 2 down the same door, so I just followed the guy in front of me to the left, not knowing that the right path was also being boarded. I got onboard and took my seat. A few minutes later someone else came to my seat with the same seat assignment. Only after comparing Boarding Stubs did I realize I was on the wrong plane!

Can't recall which airport this was at, but it was a long time ago before Gate Readers.