This didn't happen while riding, but it is a head scratcher:
Was starting the build on a nicely repainted Faema Merckx Professional. Right dropout adjuster goes in just fine. but threads on left side dropout don't want to play nice with the screw. Try another screw, same thing. Pull out my trusty 3mm tap, oil it and run it into the dropout. Get it almost all the way through and hear that terrible 'krack!' sound as the tap breaks off inside the dropout.

It's too far in (on both ends) and too stuck to tease it out, too small for a tap remover, and too hard to drill it out and retap the dropout. Now trying to decide whether to pay the $60 to EDM it or just live with no adjuster screws.

Dang.