Gremlin

A Gremlin is a wiry saboteur attracted to machinery, locks, wagons, mills, and any device whose owner trusts it too completely. It has oversized ears, clever fingers, and a collection of screws and pins removed from places nobody can reach. Gremlins rarely confront armed travelers directly; they make one essential object fail at the worst possible moment and watch what panic does next.

Essential Part: one nonmagical device, weapon, or tool within 20 ft fails unless its holder succeeds on a Quick save. Repair requires one uninterrupted round and suitable tools.

Simple Mechanism: equipment with no moving parts cannot be affected. Leaving a deliberately harmless puzzle mechanism distracts the Gremlin until it solves it or takes damage.