Riding my patrol bike around the university, I see a LOT of bikes. It's one of the things we keep track of, trying to prevent bike theft.

Being a mechanic as well as a cop, I can't help notice how many bikes I see with the quick-release levers on the hubs backwards.
This should not be difficult; they are even marked "close" and "open" in most cases. Yet it's very common.
Another thing you see all the time is bikes "cross-chained" badly; the big ring up front and the big cog in the back, or vise-versa...
Not to mention chains that have never seen a drop of lube. Often, you can hear the kiddies coming... Sounds like a threshing machine.