During a recent rehearsal my Mission expression pedal (which I use only as a WAH) failed by staying in the full-treble side of the sweep, but could be switched on and off.
After rehearsal, a glance through the hole in the body, under the pedal, revealed that the entire potentiometer had come loose - so the "rack" on the pedal could not contact the "pinion" on the potentiometer.
It's a simple fix to remove the bottom and re-attach. So simple, in fact, that a week later it failed again since I apparently put the locknut on the wrong side of things. 
Anyhow I now consider it a regular maintenance item.
Carry on!
