This might not count as a "most used part" but its still interesting.
At our kickoff this past January, AndyMark gave our team a 12v solenoid, thrown in with the other 24v solenoids that we used. We noticed this problem later when testing pneumatics, and we had to get AndyMark to change their website description of the solenoids they sold.
Proof:
http://www.andymark.com/product-p/am-0888.htm