I've just had to drill out an alloy wheel bolt because the head sheared off it... I'd torqued them all up to the correct spec, and even checked that I could undo them easily. I usually do them up tighter just going by feel with my breaker bar! Anyway I torqued them up and came back a few days later and they were all ridiculously tight. I had to jump on a scaffold pole over my breaker bar. One sheared and I had to destroy the wheel in order to get it out. What could cause this? I put some copper grease on the bolts a while back, perhaps this has had an effect?