Where do I even start...picking the worst would be very difficult, so I'll list a few contenders for bad work, or just plain dumb work...
Start with this nice Zenith pocket watch...
How to keep the ratchet wheel in place when the screw is busted off in the barrel arbor...
New barrel arbor and ratchet wheel screw were made for this repair...
How about a repaired minute wheel post on this Speedmaster - looks like someone just jammed an old winding stem in there (the part in the hole is square) and filed the post to be semi-round...
Proper repair done...
Someone was using something other than the correct tool to remove the chronograph drive wheel on this Speedmaster movement...
When the regulating pin on the Cal. 321 Speedmaster (sold by a well-known dealer) broke, the solution to repair it involved copious volumes of glue...
How about the Cal. 321 balance staff that someone decided to...for lack of a better word, sharpen?
Other end:
For reference, this is what the staff pivots should actually look like:
The balance amplitudes with this staff were excellent, but positional variation was crap - new staff required.
Braking grease inside the barrel...
Of a manual winding Speedmaster...
And this was after a factory service no less...
🤦
I could go on for pages and pages...unfortunately...