The worst thing that happen to me was in 1st edition Ad&D about a year before i quite d&D altogether until 3e. He let a character pray for teh sword of Kas. The plan was that I would "steal" teh sword and be possesed. They told me that he was praying to his god for a powerful sword. So I stand guard while he prays. Anotehr character who heard about the plan laughed because it "back-fired" I didnt try to steal the weapon so teh DM had the sword possess the otehr player and he attacked and killed me with teh artifact.

Why they thought a ranger would steel his compaions weapon I'll never know. Ironically it was because I roled played my ignorance about what the sword of Kas was that got me killed. I had a few rounds to act while it appeared and before the character finished his prayer and chanting. I could have killed him for being so evil. But i was playing a character with little knowledge of history.

How a DM could just allow a character to pray for an artifact I'll never know.