First dungeon you explored as a player character or ran as a DM? I ran the Haunted Halls of Eveningstar, a 1st level module for ADnD 2E. I made the mistake of having a PC discover a sentient blade that wanted him to kill the party. I didn't realize he'd agree. As the party left the Halls, they set fire to some grasslands that were ancient elven holy lands and for years I had the elves declare war on the Forgotten Realms over many campaigns (I hated elves). This particular campaign ended when the same player who was tempted by the sword, a druid now, miscast Web centered on his character and caught all of his allies and none of the enemies. Again, my inexperience resulted in a TPK.