A Piece of String
When I started working as a field service engineer with a company that made cheap and nasty cheque encoders I noticed that some of them had a piece of string hanging from out of the front of the machine.
The machines were based on a mechanical adding machine, the numbers were entered on the keyboard and when the cheque was pushed into a slot the adding machine went through its cycle, being stopped half way by a solenoid to allow the printing mechanism to print the amount on the cheque. The solenoid didn't always release, which resulted in a service call, so some engineers tied a piece of string to the armature so the operator could give it a tug and carry on her merry way.