If you brake really hard, you may get rear-ended -- particularly if you're driving a gigantic beast of an SUV and the car behind you is a Geo Metro. I never assume someone's doing something wrong just based on the law; I also check for other factors like who's around them, what those other people are doing, whether the street is wet or oily, et cetera.

. . . and whether the speed limit in a particular area is artificially low to increase ticketing revenue.