I have broken a few crank arms and a BB spindle once, which all resulted in "limping", i.e. riding one-legged home, but it hasn't been more than 3-5 km or so.
Other mechanical problems that have rendered my bike unridable include:
- punctures while doing short rides in the neighborhood without any toolkit on me
- rim ripping open along braking surface due to wear and causing the tube to explode with deafening bang
- broken quill stem handlebar clamp (twice on the same bike, both times while pushing hard uphill, both times stopped without crashing)
- bent fork after a frontal collision so that front cantilever brake cable was stretched tight and locked up the wheel (too tight to get it undone)
- snapped derailleur hanger
- I have broken a few frames too, might be that at least once the damage was too severe to ride on it (can't recall for sure though)