Ah yes. The lovely 'Okay look you can't break this one' plot device. My favorite. Kiddo's, tell a story of a time you broke the plot device, and the DM had to pull a replacement out of his ass, or whatever nearby available hole. Really, anytime you broke any item of importance, even without a replacement will do.

Just the other day in Vampire Masquerade: Bloodlines, my Nosferatu was snooping around the downtown apartments and noticed an item for the taking on a table against the wall. I close in to grab it, but accidentally click on a graspable object on the way and the item is knocked behind the table, never to be taken until I give up and reload a save file.

It wasn't exactly a plot device, but it did help doom them.
I ran a couple guys through a level 0 npc adventure. They found some items along the way. Well, the merchant found some items, one of which was a glowing Whetstone of Magic Weapon. Gave the weapon it was used on a +1 enhancement for one minute after it cut something. So an oil that almost couldn't be wasted. And when questioned about the glowing rock he was suddenly carrying around the merchant said it was just his whetstone and proceeded to use it on a knife, the stone turned to dust and the knife began to glow, he then proceeded to use that knife to do some whittling. Needless to say when they came across the monster with DR/magic they got murdered because nobody had a way past its DR.