When I saw the topic the thing I thought of was to put the rod into the hilt of a sword and not tell the user. All of the sudden, when you want to hose him, you just speak the "magic word" and boom, no more useful sword.
I don't think it would quite work RAW, but it could be a fun plot point. Bob the archmage had given his not-so-trustworthy follower, Tim, a sword that when a certain word was spoken would freeze the sword in place. Many years later Lisa (PC or NPC) gets the sword and Laura (PC or NPC) has learned of the secret. Maybe Laura needs to find the word to stop Lisa, or maybe the DM has given out a way too powerful weapon to a PC and wants to reign things in with the secret weakness of the blade.
Mark