Did a stint as a dispatcher (turnround coordinator) in the mid 80s for a summer. I was doing a JAT 727 to Pula (or similar) at the end of Pier B at MAN while a colleague was doing a Britannia 737 to Spain on the adjacent gate. Unfortunately the 2 gates used the same exit out of the pier (think it was gates 14 and 15 before it got changed to accomodate the 380), and although we positioned someone between the 2 aircraft to try and steer people to the right aircraft, I still ended up with 2 Britannia pax sitting on my 727. This came to light fairly easily as the correct occupants of the seats showed up and wanted to know why someone was already sitting there!
We had been under the impression that those 2 gates weren't normally allocated for flights departing at similar times, but obviously that had been forgotten on this occasion.
Going off at a tangent, the mention of JAT reminds me that at that time east European crews were notorious for getting off as early as they could get away with so that they could nip to the shops in the terminal for such things as chocolate which were scarce back behind the Iron Curtain.
Normally they left the most junior crew member behind to "supervise", but would usually come back before most outbound pax had boarded. On one occasion however I had a JAT 727 full and ready to go, but with no flightdeck crew and just a young stewardess down the back. The loaders insisted on removing the front steps as they had to go to another job, so when the missing crew returned 5 minutes after scheduled departure time they were forced to board at the rear stairs, and do a 'walk of shame' to the front!