A person that has to do exactly what you tell them not to do, usually a smaller child, or annoying older child, or an adult with childish tendencies or ADD. This is not limited to just crossing lines, and also involves situations much like the following: "don't touch my hair" (at this point, the "line crosser" would poke the persons hair while saying "tooouuuchh") line crossers also have tenancies to be unimaginative, and do things like finding something, and naming it "Bob", "Fred" or "George" because they can think of nothing more original. (nothing against those names of people, just random objects) it is perfectly legal, and in fact, encouraged to smack line crossers when they do what they do, because it will eventually show them that in the real world, they will be beat up by people bigger than them.
PERSON#1: "Ok, i am on this side of the line, *points to line in sidewalk* now don't cross it please"
LINE CROSSER: *puts a toe/finger across the line* "look, i crossed it!"
PERSON#1: *smacks line crosser* "wtf did i say?!?"
Some one who is known for crossing lines consistently without caring. Can include telling people off, saying things you should not around other people, doing things to other people/other people's property.
A line-crosser can be someone who upper decks someone's toilet or putting an air horn in someones ear.