Being a bit of a gearhead with engines dating from as early as 1921 its usually a one off nut or bolt to some obscure old thread standard that I end up dropping on the workshop floor that just happens to be the roughest bit of floor that dropped items just seem to evaporate into.
When I worked for the MOD it was the ****** nuts and bolts that held the hypertac connectors together that drove me crazy. Nearly as bad as BA nuts and bolts.