The left bicycle pedal screws in backwards
Try to undo the left pedal the usual way and it tightens instead.
Category: Getting about
Be the first to vote on this one.
Feature. It stops itself unscrewing as you ride.
The right-side pedal has a normal right-hand thread. The left has a left-hand thread. Pedalling causes a motion called precession, which would loosen a normal thread on the left side; the reverse thread tightens it instead.
Source: Bicycle pedal, Wikipedia.