Arriving at the Sacramento Valley Station the other morning to catch my Capitol Corridor train, I saw my friend and fellow multimodal bike commuter Daniel sitting near his crankless bicycle. Upon closer inspection, I noticed this: the spindle on his bottom bracket (where the crank arms attach) had broken on the non drive side. Wow. And I thought I'd seen everything: broken spokes, broken frames, broken derailleur, broken rims, a broken stem (!)...but never a broken bottom bracket.
Luckily, Daniel was uninjured. Equipment failure is a big cause of bicycle crashes. Most equipment failure can be avoided by frequent maintenance of your bicycle. Daniel confirmed that his bike had been creaking for some time. So he was fortunate. If you hear unusual noises, inspect your bike thoroughly. Remove your cranks and inspect the bottom bracket. Look a the frame, especially the chainstay on the drive side. Examine your chain, fork, brakes. Many cyclists always do the ABC Quick Check before every ride. Check that your tires are properly inflated (A="air"). Check that your brake cables and levers (B="brakes") are fully engaged. Check that your crankset, chain, and cassette is in good working order (C="crankset"). And make sure the quick release on your wheels and seatpost (if applicable) are closed and tight. Doing the "ABC Quick Check" before every ride is a good way to detect possible injurious mechanical failure before it happens.
If you think you will do most of your own wrenching, my advice is to buy a complete bicycle tool kit. It's much cheaper to acquire a complete kit than to do a one-at-a-time piecemeal acquisition of necessary tools (which is how I built my tool kit.) For major overhauls, don't be too proud to take your bicycle into a shop. I've done everything mechanical on a bike from building wheels to overhauling a bottom bracket (old kind, pre-cartridge), and I'm not ashamed to leave my bike with a skilled mechanic. So take your bike to a shop if necessary. That's why they exist, to provide professional service to keep your bicycle running well.
The reality is that things will eventually wear out on your bike, just as parts wear out on an automobile or a dishwasher. It's not a big deal, as long as you pay attention to your bicycle's condition, and make necessary repairs.