Am I gonna need to call Juan Williams in here for backup, or can we let this be?

^ exactly

I did about seven years ago. I was flying LAX-BOS on AA and got to LAX pretty early. When I went to the gate listed on my BP there was a long line to board and I walked to the front as I had AA status. The machine beeped and said "wrong flight" but the agent told me to board. I sat down and eventually a girl sat next to me wearing a Yankees hat. I said "that's a dangerous hat to be wearing on a flight to Boston" and she gave me a funny look and said "this plane is going to Miami". I ran off the plane where the agents said "oh we were looking for you, you're actually supposed to be on a different plane".

Luckily the BOS flight was at the next gate and hadn't yet departed. The whole ordeal did mean I had to gate check my bag. My fault for not double checking the monitors when I boarded.

Never happened to me but on my last trip through ORD I arrived and left through gate G2 (IIRC) that split into A/B about half way down the jetway so I can totally see how it could happen if two planes were boarding at the same time.

A couple years ago while waiting to leave PHX (I think) a GA came onto the plane looked at a guy's BP and from the look on his face must have told him he was on the wrong plane because he looked at his watch, grabbed his stuff and quickly got off the plane.