Cycling is so hard, and our bikes don't always cooperate. One major source of struggle between us and our steeds is the drive train. Darn chain!
Multi-speed chains are laterally flexible so they can move between adjacent gears. But only a couple of the sprockets (rear gears) align straight with the chainrings (front gears). So you end up riding a lot with the chain "crossed up," that is, you force it into a slight twist between, say, the outer chainring and the inner sprockets. That in turn causes a few problems. Just for starters, the chain will wear out much faster than if you keep the chain straighter; all of the force is concentrated on one chain-link bearing surface instead of both. It is better to choose gears more in line, like the inner chainring and inner sprockets. Or the outer ring and outer sprockets. That way the force on the chain links is lower and it won't wear out as fast.
Another problem with ''crossed-up chains" is that the lateral load from the twisted chain encourages it to jump off the gear teeth, and even snap under heavy loads. That can be a problem in some races, like Cat's Hill. "The Wall" there is only 15-20 seconds long, but a very steep 20% grade. Lots of guys break chains, or have it skip over the gear teeth, sending them sprawling to the ground. I bet a lot of them try to keep the chain on the big ring and that exacerbates the problems from high chain loads. I've been using a 42-tooth small chainring in races like that instead of a 39-tooth; it's still low enough, but with nice, tighter ratios.
This chain-jumping can also happen in typical sprint finishes where guys force as much as 2000 Watts of power into their pedals. You need to be sure your chain can handle the load. Keeping your chain straighter will help a lot. So, straighten up and ride right!