Our DM (the bastard) set one of our player's against me because I had become a bit cocky. The PC eventually killed me in mysterious circumstances, and to cover their tracks they had the same guy that killed me resurrect me shortly thereafter so I wouldnt suspect foul play. I found out months later when I found my gear when searching his body after he died - very sneaky.
As a DM, I had an army against my group (with lots of NPC's). It ended up being a siege, and my group escaped after blowing the castle's powder store and escaping thru a hidden tunnel.
I also loved using the scenarios pre made in Dragon magazine.
Our DM gave us an Uber-equipement planning that we lose it in the meantime.
Best way to make PC's lose stuff is have them on a boat and capsize it ;-)