Had a rear derailleur rip itself off from a stick caught in the drivetrain. I shortened the chain and tried to run it as a singlespeed, but the chain kept trying to climb up the ramps into the larger cogs so it kept jamming.
I also once had a chain break and managed to put it back together (sort of) with a rock. That got me far enough to find someone working in their garage who I could ask to borrow some pliers to improve my repair, and that got me home by soft pedalling and not shifting.
I once had my frame break at the junction of chainstay and rear dropout at the turnaround point of a popular 50 km out-and-back route, and every time I tried to pedal the tire would rub up against the chainstay, so I got my ridding buddy to tow me with a piece of rope I found on the side of the road.