Something or someone described as errant has gone astray or done wrong by going in an unexpected direction. An errant bird might end up in northern Canada while his friends fly to southern Mexico for the winter.
Although errant is commonly used as a synonym for "misbehaving" or "naughty," it also refers to things that are just out of place. An errant bomb can hit a house instead of an army base, and an errant lock of hair can get in your eyes. Errant can be intentional, as in "the errant boys skipped school to go to the movies," or unintentional, as in "an errant snowball hit your neighbor instead of your arch enemy."