V-Brakes rubbing
Apart from wasting power, gritty brake pads can wear through a rim very quickly, so fix them!
Disk rotor bent
Just dropping the bike on its side can bend the rotor. Crashing it can bend it severely. Make sure you take your time to get it straight again.
Hydraulic brake hose broken
It's surprisingly easy to rip out a hose by catching it on branches. It's often possible to put it back together again afterwards.