Here's my personal "do it once and learn your lesson" valve core failure, Presta edition.

While topping off tire pressure in the garage before a ride:

1. Attempt to unscrew the valve nut.
2. Unscrew the valve core instead.
3. Valve core becomes projectile that ricochets through the garage.
4. Try to find where valve core landed, but fail.
5. Have no spare valve cores.

Lesson: Always tighten your valve cores with some needle nose pliers.