Riffing off James Raggi's idea.
Here's one for 3e/Pathfinder DM's:
The local temple known for selling wands of cure light wounds has been infiltrated by a cultist of the death god. The wands seem to work as normal, but 24 hours after receiving his first heal from it, a character takes 1 point of Constitution damage per cure he received from it in the prior 24 hours.
I Was Wrong (The 1930 Hobbit)
2 days ago