NASA fastener design manual

[NortonDommi]

I see in the N.A.S.A. Fastener Handbook that as with life the universe and everything the answer is 42. That is an astounding 42% of joint failure is due to installer error! 14% wrong preload and 28% improper assembly.
A common cause of both that I have seen regularly is dirt. I see people pull things apart without cleaning the area first and then give a cursory wipe with a dirty rag before assembly, or more correctly this is what I find has happened when doing repairs. A collection of old blunt taps are ideal for cleaning tapped holes and the smaller sizes can be run in and out very quickly with a reversible air driver,(drill), an air nozzle with an extended reach can be made to blow out debris. Studs can have a die nut run down them or wire brushed and a few minutes at a wire brush on a bench grinder will clean up various bolts, in the field shaking fasteners around in a bucket with solvent will usually suffice.
The worst thing about diagnosing a failure caused by lazyness or stupidity is that you usually know the problem was caused by someone who should have known better.