Originally Posted by
pyroguy_3
Could somebody explain how overtightening can ruin the arms? Can it deform the metal? Or is it more a matter of stripping out the spindle?
It can cause the crank itself to crack at the taper. Like this
http://pardo.net/pardo/bike/pic/fail/FAIL-005.html
You'd have to put a lot of torque into tightening it, but it's not totally uncommon.