I guess no one has had an idea for a great encounter in quite some time.
Since 4E I have DMed only two games and the last was a TPK. I used DCC#11 and the party was wiped out in areas 1-3, 1-4, and 1-5. At the beginning of a long corridor the party comes to a trapped portcullis. After getting by they find a secret door and the undead dwarves that were stabbing them through the wall when they were stuck at the portcullis. With the battle going against them the PC's decide to run down the corridor deeper into the dungeon. That did them in. The corridor was trapped three more times.... TPK
This idea is not mine. It belongs to Chris Doyle(who was nice enough to sign my copy at GENCON a couple of years ago) but could be used in any dungeon crawl as a great encounter. This was one of the $2 modules.