I never did beat the game, but My most memorable experience was once when I was messing around in wizard mode(a sort debug/creative mode to try things out in) and summoned a high level demon for grins and giggles, which promptly killed me. I did not think much of it at the time. but nethack, when you die, writes a bones file with what you had on what level and what killed you. it normally lets you find what killed you and a chance to get your stuff back. But now my nethack install is tainted with Juiblex, the faceless evil, on dungeon level level 2. I probable could have fixed it but it made the game hilarious, every play through containing a mad dash through level 2 and fervent usually unanswered prayers that the infernal thing would not follow you to the next level.