Broke a rear axle sometime during the first 1/2 of a 200km brevet. Noticed it at the turn-around checkpoint when I took the rear wheel out to fix a flat. Since the QR skewer had held everything together til that point I figured it would hold to get me back and it did.
Dropped the chain and it wrapped around the cassette somehow and pulled the rear derailer inward and bent the derailer hanger and broke the derailer cage pivot post. This was during an 11 day tour and I was ~25 miles from any city with a bike shop. I happened to have packed a 10" adjustable wrench which I used to bend the hanger back (steel frame) and I limped into town and got a new rear derailer.