I did it once on AA at DFW several years ago. Late flight in, very tired. Waiting for my flight to board and was watching a movie on my laptop, not paying attention. They started boarding, I got on the plane. I am dozing pretty well while waiting to leave. As we are about to takeoff, pilot does an announcement that we were 2nd to take off and would be in San Antonio in about 35-40 minutes. Unfortunately, I was heading to Austin. It was too late to say anything, but afterwards I found out they moved the Austin flight two gates down while I was tuned out and I was not nearly aware enough to notice I boarded the wrong flight.

I did a one-way car rental to Austin and just added it to my expense report.

On a side note that is kinda related, I was once on a flight (AA again) where two people got combative with the flight crew when they found out the flight was to Ontario California, not Ontario, Canada. Unfortunately, we had already taken off. We had to land in El Paso to let off (and arrest) the passengers in question.