Childhood cartoon characters that have suffered through major trauma and have transformed into a twisted version of themselves. There is a very short list of real BEFs, while many that are added by fans are unworthy of the title.
Bipper, a Dipper Pines possessed by Bill Cipher is one of the BadEndFriends.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.