"Pass me the blue one." said Quinn.
"Are you sure?" asked Elise. "It's just, you were sure about the last two combinations, and that ceiling got closer both times."
"It's a gamble, I agree," replied Quinn, "but whatever's behind this door has got to be worth it."
Elise passed Quinn the blue stone which he placed carefully in the final slot. As the grinding began, Quinn looked to the door while Elise looked up...
"What do you mean the vault is empty?!"
"Just that, sir. It's empty."
"There was a half tonne of gold coins in there! Did you see a cart? Any orcs or ogres? A dragon?! It's not like it just got up and walked away!"
The adventurers hear a noise and turn, only to see a bottle clatter down the street. Shrugging it off, they continue on their way, blissfully unaware of the hulking mass of junk taking shape behind them...
On a location with numerous webs, and at least one big spider, there is a something inside a cocoon. It is humanoid in shape, still moving. If the heroes free it (not before they kill or drive away the spiders), they meet a ... zombie!
The poor zombie wandered the dungeon alone, and tried to kill a big living creature (= a spider). The spider used the usual treatment, even if this victim did not look tasty. The zombie can be easily killed as any other zombie. It got but several doses of spider-poison, so can be something worth if it is extracted. You can mention to a druid or ranger the fact the spider had no poison anymore.