Yes, has happened to me on Air Asia.

Two aircraft parked next to each other with mobile staircases being used and passengers going through a gate and walking down a flight of stair cases to the ground level parking bay.

There was no ticket agent below to direct passengers to their respective aircrafts and it was not surprising 1 passenger got on the wrong plane.

The other reason it happened was the FA's were too busy chit chatting and did not check passenger boarding passes as they came up the stairs to enter the aircraft.

Passenger realized he was on the wrong aircraft when aircraft was pushing back and pre-take off announcement was made announcing flight number and destination.

We had to be pulled back to the bay and pilots had to switch engines back off.

Pilots were not best pleased to say the least and an argument ensued between passenger and leading attendent as the aircraft the passenger was supposed to have boarded was pushing back and already leaving.

Stupid incident which should not have happened. Flight attendents blamed the passenger which I thought was not correct.