the best one for me lately is the people who take off the helmet or just never wear one to ride the MUP around here.
in my opinion, on a nice sunny day, it is WAY more dangerous than any street and the only place i have ever tangled with a car is in one of it's intersections.
yet most people seem to think, oh i am on a MUP safe away from the cars...
some moroon on rollerblades can make you crash just as hard as a car.