A typical wand with offensive capabilities (magic missiles, fire, fireball, lightning bolt) that was either damaged in combat or made just under par. When the wand is discharged, there is a 1 in 4 chance that it fires an additional 1d6 charges simultaneously or in rapid succession. Wands that shoot fire may incinerate innocents and friendlies, or burn whole buildings and even villages down. Those which shoot fireballs have a considerable radius, and lightning bolts that bounce upon contact with ground and stone can cause catastrophic random collateral damage. Those who have paid large sums for such a device may go seeking a refund, possibly even retribution.
Bag on a Stick
Very simple gag but a great one, since it can be used multiple times over, even in the same adventure. Great for tribal natives gone restless and humanoids, but anyone can have set this up. Just what the header says, a simple bag over a stick stuck in the ground or floor.
As GM you can place the bag on a stick anywhere, in a floor crack the heroes have passed before, outdoors in a clearing or path, or at the edge of the PCs' encampment the following morning, what have you. Place anything on the stick - a coiled yellow viper angered by the bag removal, mini crossbow w/poison, transdermal hallucinatory drug dusted on the bag, yellow mold colony, an NPC ally's head, a weapon, scroll tube or satchel, what have you.
The idea is to build tension and/or stall for time/distract the party. Provided it's used properly, you'll be amazed at how paranoid players will get from this simple gag.
Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or "bounce back". The grater the magic, the more it should cost the character physically, emotionally, and spiritually.