OK - what is the weirdest fault you have come across?
I have on my bench at the moment something which at first hearing sounds pretty strange... It is a twin fusee bracket clock with (unusually) 1/2-hr rack strike. The only fault was the single blow due at 1.30 which consistently came at 1.36 instead...
When I checked it out the explanation was far simpler than expected. The 1/2-hr strike is limited to one blow by the rack hook not being lifted far enough to clear the second & subsequent teeth on the rack, but only the short first one. At 1.30 - uniquely for the 1/2-hr strikes - the rack tail also falls into contact with the snail. It turns out that it was hitting the snail on a marginally high spot so the gathering pallet tail was just not clearing the pin on the rack. By 1.36 the snail was back to the correct height and the train was free!
A quick tickle with a file and the problem was solved... and yes, I could also have solved it by moving the hour wheel round one tooth on the motion-work pinion since the hour hand is on a cylindrical collet, but if the snail has been badly formed in the first place I see no argument for not correcting it.