Derailleur hanger bends

Symptoms include rear shifting "skipping" or refusing to go into or stay in gear. Most modern hangers are designed to do this in order to protect the derailleur.

Front derailleur breaks

Hard to do, but if you bend the guide plates your chain won't stay on the chain ring. Tips for reparing or removing the derailleur.

Rear derailleur breaks

Normally caused by a stick in the spokes or a trailside rock. It's unlikely you'll be able to fix this on the trail, so it's time to go single speed!

Gear cable snaps

The shifter feels wobbly, the chain is in the biggest cog and you have three more hills to climb before getting home.

Freehub freewheels in both directions

It's the strangest feeling to pedal forwards and not get anywhere. If it's just started happening, you may be able to temporarily fix it.

Pedal breaks 

Normally pedals don't break so much as fall apart. So long as you have the bits, you can fix it. If you've lost the parts, you can still ride out.