Area 21. The Agitated Chamber
Past Area 20, the hallway runs for 100 feet and dead ends. There's a secret door on the north wall of the hall about 60 feet along that leads to Area 21.
http://www.wizards.com/dnd/images/ToH_Gallery/ToHGraphic21.jpg
Area 21 is a 30 foot square room full of junk. Rotted sofas, throne-like chairs, chipped vases, dented urns, small tables, braziers -- all jumbled up everywhere. There are 6 locked trunks and 24 locked coffers "amidst the general havoc." "Rather plain" tapestries hang on the east and west walls (in the illustration, they're decorated with little fishies). A door is visible in the northeast corner of the room.
First. The door to the northeast is a fake.
Second. All of the trunks are empty. The coffers mostly hold angry, poisonous snakes. (50% of the coffers hold asps. A third of the coffers hold 8d10 platinum pieces. One out of six coffers holds some low value gems.)
Third. The floor. When the PC's step on the floor, they "set a mechanism into motion." After that, there's a 50% chance each round that the floor of the room will "jump and buck up and down violently." Anybody standing on the agitated floor has a 1 in 3 chance of falling down and taking a point of damage.
Fourth. Those "rather plain tapestries" with the fishies on them are actually "specially anti-magic treated creations of green slime and brown mold." If the tapestries are torn, they turn into green slime and cover everybody within 10 feet of them. Being doused in green slime destroys you. You turn into slime yourself and can't come back. Luckily, fire kills green slime! Unluckily, burning the tapestries turns them into brown mold, which eats fire and radiates deadly cold. The mold will drain 4d8 hit points from anybody who gets within five feet of it. The module says "it gets worse from there," and I dimly remember that brown mold maybe does more and more damage as it grows, but neither the module nor the Monster Manual really says that.
The PC's can touch the tapestries without fear. But if somebody is holding a tapestry when the floor goes berserk, there's a 75% chance the tapestry will tear and unleash slimeaggedon.
Fifth. The way out is a secret door behind the tapestry on the west wall.
IMO: Not counting spiked pits, this is the first insta-kill deathtrap that the PC's must negotiate. It can't be bypassed or ignored.
So what clues or hints do we have to help get past the Agitated Chamber? AFAICT, somewhere between none and almost none. Acererak's riddle says "beware of trembling hands ." Is that a reference to Area 21? If so, it is a piss-poor clue. First, the riddle gives no clue to connect "trembling hands" with anything the PC's see when they get to Area 21. It makes no reference to tapestries or fishies or rotted sofas or anything else. Second, calling the threat in Area 21 "trembling hands" is a stretch. The problem isn't that the PC's hands will tremble. It's that the ground will go buck wild and make them accidentally tear the magic tapestries. It's more important here for the PC's to have steady footing than steady hands.
Nor do I see any warnings or clues in the room itself. It's full of junk just like Area 19, and there's nothing specifically ominous about the junk. I've been kidding about fishies, but the module specifically says that the tapestries depict "weed-grown rocks and green and gold tan scenes of undersea life," which isn't much of a warning that they might turn you to goo.
Further, the actual description of how the floor works is opaque. The module says that each round the PC's remain in the place, the DM should roll a d6. If an odd number comes up, "on the next turn the floor of the room will jump and buck up and down." "Turn" is a term of art in 1E that means 10 minutes, right? So does that mean the PC's have 10 minutes to explore before the place goes nuts? I don't think so, but I'm not 100% certain. And once the floor starts rocking, how long does it keep going? The module is silent.
Finally, determining whether or not a PC is holding onto the tapestry when the earthquake sets in is probably going to take a fair amount of DM discretion. "I inspect the tapestry." Am I touching it? Am I holding it? Let's fight about it!