This one is easy. It was around the time I started playing AD&D and I had a very good DM. Probably the DM I emulate myself on most of the time though he was pretty lazy getting games together. I can’t remember which character this occurred to but I have a feeling it was my half-orc fighter/cleric that was waylaid by a bandit after he had fallen behind his own group.
The bandit was just after money apparently and turned out to be very capable at his job. Before too long at all Amphora found himself unconscious, and I was sweating the fact that he may be dead.
Later in the gaming session though the DM mentioned that Amphora came walking up the path to the rest of the party and I was relieved. It even seemed that the bandit was scared off as though I was bruised and disheveled I was OK and rejoined the group. Everything was awesome again and I happily played out the remainder of the session until I went to cast a spell to save the group. Instead the DM took over and all of a sudden Amphora transformed into a creature that attacked the person I was trying to save.
I was in shock a little and the rest of the party a bit taken aback. I had been defeated by a Doppleganger and it had replaced me in the party. The group defeated it and went back to find Amphora unconscious and bound, but it was so totally the most impressive surprise I have ever been subjected to in a game to this day. Deceptively easy to pull off too! I have had my own share of doppleganger moments in my own games over the years and it never fails to fire!