So, our group is running the Bar-Lgura's lair in "Here there be monsters". Much of it is going on fairly easily, with the PCs surviving the rockfall trap (no problem) and figuring out the "door puzzle" quickly. They're a little worried about the obvious relation to Demogorgon (in fact, two of my players are getting VERY apprehensive), but all's going well.
Then they get into the "Candle Room".
I fudged things just a little bit to make the puzzle easier (so that the PC that lit the candles didn't have to sit in the throne; any PC sitting in the throne after the candle was lit could see into the mirror). Once the Paladin got trapped into the mirror and the group had a hard time subduing him (the Paladin player loved having the chance to beat on his fellow PCs, and hit our Goliath fighter for a dear amount of damage), the group realized they weren't going to put up with that again.
This time, they bound the paladin up after stripping him, so that when he went feral the second time, he was already bound and easy to maintain.
No problem.
Once both mirrors were glowing, they decided it was time to touch them again. So, once more, the paladin (naked and bound) heads towards one of the mirrors, and touches it - teleported instantly into the Bar-Lgura's lair, TIED UP AND UNARMED!
Naturally, even though we still had 45 minutes of game time, I ended with "that was a mistake... session's over for today". Gotta love cliff-hangers.