Umd

[roguerouge]

Does failing a UMD roll mean that you use up a charge on a wand or erase the spell on the scroll.

 

[Merkuri]

We've always played it that failing a UMD roll means you failed to trigger the device at all, so you do not use a charge. It's like trying to put the wrong key in a car ignition. You don't use any gas because you never successfully started the car.

And, in the Activate Blindly portion of the UMD description the SRD says:

If you fail by 9 or less, you can’t activate the device. If you fail by 10 or more, you suffer a mishap. A mishap means that magical energy gets released but it doesn’t do what you wanted it to do. The default mishaps are that the item affects the wrong target or that uncontrolled magical energy is released, dealing 2d6 points of damage to you. This mishap is in addition to the chance for a mishap that you normally run when you cast a spell from a scroll that you could not otherwise cast yourself.

Since it mentions scrolls, I believe it goes for wands and similar items as well. I'd interpret that as you do not use up a charge unless you successfully fire the item or suffer a mishap.

 

[Psimancer]

Totally agree with Meruri; failing a UMD check should never expend a charge.