THE MISTAKE: When one student's shifting suddenly went wonky, he thought adjusting the derailleur's cable and limit screws would do the trick. But during a ride, his chain fell off his big cog and flew into the spokes, breaking six of them. Then the derailleur broke off, spun around and exploded against the frame.
THE FIX: Aside from cable stretch, which occurs naturally (especially when a cable is new), a bent derailleur hanger is the cause of most shifting problems. To diagnose, shift to a middle cog in the rear, then hop off your bike and squat behind it, holding it upright by the rear wheel. Look at the cage of the rear derailleur (the part that holds the two pulleys) and compare it with the cogs. They should all line up vertically. If it looks bent inward or even slightly twisted toward the wheel, the derailleur hanger needs to be straightened or replaced. To prevent the aforementioned catastrophe, stay out of the largest cog (your easiest gear) on the cassette until you can get to a shop.