In 1990s my grandfather accidentally boarded the plane flying to Frankfurt instead of Dusseldorf and got there. They were departing nearly at the same time, so it was a matter of walk to a wrong plane. He just checked after boarding with flight attendants that it’s indeed THE flight to Germany. Thankfully he had enough relatives all over Germany, so someone could pick him up there too.
Unimaginable situation today because of ID checks.
Still quite possible, just boarded a flight that had two gates past the final check, you could (not entirely easily, but quite accidentally) switch if both were active.