What's your most expensive oops?

[Alan]

A buddy of mine tightened a bolt way too much and snapped it right off. The re-tap ended up costing more than the original part!

What torque-related blunders have you faced?

 

[Cuttingranger]

I once snapped a spark plug because I tightened it too much, had to take off the head to fix it which ended up costing way more than the plug itself.