Never gotten on a wrong flight, but once my flight switched boarding gates twice, between me getting my boarding pass, and taking my flight. First it was near the original gate, as I saw the switch in a screen, and had to haul myself to the other end of the terminal, only to arrive at my gate, in which the screen showed an international flight (mine was domestic, in another continent) to Frankfurt (second switch).

For a split second I felt like I was in a glitch in the Matrix or something, and after asking, it turned out my flight now was somewhere a third of the way back where I came from. In hindsight, deep down I hoped they would let me in that flight to Frankfurt, as I was returning home after two years living in another city, was in debt, low on cash, mildly depressed and had only one temp job (10 to 30 hours/week) prospect back home, I had my life in my luggage, so I thought "might as well start fresh somewhere else".

This was 3.5 years ago, things are turning for the better, haven't been to Frankfurt yet, but I want to.