Wasn't me, but I saw a character drown because we all forgot he was wearing a tow line. We needed to explore an underwater cave back in AD&D, and so the thief was volunteered. We tied a rope around his waist, with instructions that if he got into trouble, he should tug on the rope three times. He got into a fight with an underwater creature, and although I don't remember what, it was something he should have been able to handle pretty easily. However, the DM was obsessed with this "critical hit" chart he acquired, and the creature's last blow was a crit. Since the effect rolled didn't make sense, the DM decided the rogue expelled all his air instead. He killed the creature with his next attack, then frantically began swimming back. In all the excitement, everyone completely forgot about the tow line, which should have allowed us to pull him back in time. When we realized it well after the fact, the DM pointed out that the character must have panicked and forgotten as well.