One of the more common serious mechanical breakdowns on a mountain bike occurs when the rear derailleur goes into the spokes of the rear wheel. Although this is usually caused by a much more minor problem, the results can be catastrophic.
So why does it happen and what causes it?
The answer is rarely maladjustment. Unless you or someone has been fooling around with your hi and low adjustment screws on the rear derailleur.
The most likely cause is a bent derailleur hanger. The derailleur hanger is the usually replaceable and removable piece of the frame that the derailleur screws into.
When the derailleur hanger gets bent it misaligns the whole derailleur system. This is usually exhibited by sudden poor shifting behavior. In most cases the derailleur hanger and the derailleur gets bent in towards the wheel.
The problem may not seem serious until you shift into your lowest gears with the rear derailleur. If the derailleur hanger is bent in, this shift will drive the lower pulley of the rear derailleur right into the spokes of the rear wheel followed by a lot of bent metal and sometimes expensive repair.
So why does the derailleur hanger get bent? When you crash or lay your bike down roughly on the drive side (right side) of the bike it can easily hit the derailleur and bend the hanger.
The hanger is designed to bend before the frame or the derailleur is damaged. It is much cheaper to replace a derailleur hanger than a derailleur or frame.
So how do you stop this from happening? First, try not to lay the bike down on the drive side. Second, if you crash or if your shifting is suddenly behaving poorly, slowly shift your bike into the low gears near the spokes while not on the bike. Check to make sure the lower derailleur pulley is not too close to the spokes. If it is really close it may go in over a bump or under harder pedaling.
If your hanger is bent, you will need to go to a shop to get it bent back into place or replaced.