I have a similar story, but I have to give a bit of background first:

I was on a LARP once, that played in a post apocalyptic world where the Russians had taken over the area. We were playing as a resistance cell, secretly undermining their rule. Propaganda leaflets, secret meetings, sabotage, assassinations, covert actions, attacks, ... Fun times.

Anyway, to enter the LARP you had to go through a Russian checkpoint. That was the first thing any character had to do. The 'guards' harassed you a bit, you had to go through insane bureaucracy, standing in line... It kind of set the tone for the whole event. Well, there was my friend, playing in the same resistance cell as me, going through the checkpoint.

The guard asks for his papers. My friend grabs something from his pocket and gives it to the guard. It wasn't his identification paper, it was a resistance propaganda leaflet we made for the event. He had taken it from the wrong pocket. He was executed soon after.

It wasn't that bad, of course. He changed clothes and returned as an other member of the resistance, but that was probably the quickest 'start to death' in any LARP.