Bent Derailleur Hanger
Many bikes have replaceable derailleur hangers, which are designed to bend or break rather than ruin the frame if the bike tips over or crashes. Sometimes, the derailleur hanger just gets tweaked, which upsets the alignment of the rear derailleur to the cogs and causes poor shifting. In the worst case, if you keep riding, you can shift the rear derailleur into the rear wheel.
To straighten a bent hanger until you can get it replaced (or in the case of nonreplaceable hangers, professionally aligned): Shift the derailleur to the smallest cog. Put the proper-size hex wrench in the bolt that mounts the derailleur to the frame and, with light force, bend the hanger until the derailleur aligns with the cog. Then, slowly shift to the second-largest gear and make sure the derailleur doesn't contact the spokes. Adjust the low limit screw so the derailleur won't go past this cog; this prevents you from getting into your lowest gear, but it's insurance that you won't ruin your wheel, derailleur, and frame—and possibly crash.
Cut Tire or Ripped Sidewall
When the tire tears, the inner tube usually pops through the gash then explodes. To fix this, besides installing a new tube, you need to boot—or reinforce—the tire. Anything that will withstand the pressure of the tube in the tire will work as a boot.
I like Mylar wrappers from gels and nutrition bars; other things that work are duct tape, paper money and old sections of tires. Locate the cut in the tire (and mark it somehow if you can, for easy reference), then remove one side of the tire from the rim and replace the punctured tube with a fresh one. Put enough air in the tube to give it shape, then tuck the boot inside the tire so it covers the hole. The semi-inflated tube should hold it in place. Press the rest of the tire back on the rim, then inflate enough to hold your weight and allow you to ride. Don't forget to replace the tire when you get home.
More: The Anatomy of a Bicycle
If a pedal isn't installed tightly enough, the threaded portion of the spindle starts to wobble in the crankarm. Don't ignore an unsteady feeling at the pedal—if you keep riding, the rocking motion can destroy the threads and round out the hole, resulting in a trashed crank, a big repair bill and possibly an emergency call for a car ride home. To prevent this, all you have to do is snug the pedal into the arm.
But even if the pedal falls out of the hole, you might be able to get home—and save the crankarm. If the hole is still round and just the last few threads appear to be messed up, thread the pedal all the way into the hole from the back side of the crank, then remove it. This might recut the damaged threads enough to let you install the pedal from the correct side. If so, cinch the pedal down and get home without hammering—dental work is even more expensive than a new crank. Then hit the local shop to get the problem evaluated.
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