Finding the "perfect" crank on a used frame. Pull the bolts, start the puller, feel significant resistance, stop, apply penetrant, wait. Try again... and watch the puller "pop" out, taking the little aluminum shavings of the former threads with it.
And many years ago getting in a hurry installing a bottom bracket, and realizing that I cross-threaded the fixed cup. Put it back in straight with copious amounts of blue Loctite, and it seemed to hold. At least for a while.