I think I wrote about this once in a prior thread, but it still haunts me as one of my worst days of DM'ing.
We'd been playing for a while, and the party entered the final chamber where an evil half-orc cleric was positioned on a small platform about 50' across a flooded chamber. The party had made noise by setting off some previous traps, so the cleric was ready for the fight. Being a half-orc, he had darkvision, so he could see when they approachd. As the first PC entered the chamber, the cleric summoned a fiendish giant crocodile and sent it at them.
The rogue declared he was sneaking in. His approach up the hallway was at a slight angle to the cleric's position, so he did have some cover to setup his sneaky approach. However, I forgot that he had no light source to work with nearby - just a torch quite a ways up the hallway. Stupidly, as he entered the room, I let him roll to spot the half-orc and he did. (Mistake #1).
The rogue then decided to get his bow ready for the next round. The rest of the party enters the room, and begins to fight the croc.
On the next round, the rogue takes a 5' step into a wide open square, and takes 2 shots (using rapid shot) at the enemy cleric. Both arrows hit. Despite being over 50' away, he rolls his sneak attack damage for each, and I don't think twice about it. (Mistakes #2, #3 & #4). First problem is that these shots were over 30' away, and thus were not eligible for sneak damage. Second problem was that the rogue would become uncovered after his 1st shot, so even at a closer range, the 2nd shot wouldn't count as a sneak attack. Third, his position gave him no cover or concealment, so he wasn't even able to hide in the square he attacked from. The enemy cleric was not flat-footed, so there was no chance of sneak damage at all from a ranged attack. Triple Doh!
He totalled up the damage from the 2 arrows, and it was easily enough to kill the priest. I said he was dead, and they finished the adventure shortly after that. I was really shocked how easy this priest had died, figuring it was going to be a very difficult climactic combat with all the spells he'd be able to fling at the party stuck across the pool of water. It wasn't until a few hours later I realized how badly I'd messed things up. Part of the problem was DM-fatigue, but that was still no excuse for that many errors.
Edit: Forgot to mention one other error. When the rogue took his bowshots, he was in a square threatened by the giant croc's 10' reach, but I totally forgot about that as well, and never rolled an attack of opportunity. So I really hosed up this particular encounter on several levels...