In the first season episodes Fallen Angel and Fire, a doctor and Scully (respectively) make references to "5th and 6th degree burns". There's no such thing as a 5th or 6th degree burn. 1st degree burns involve damage only to the epidermis. 2nd degree burns extend through the entire epidermis and part of the dermis. 3rd degree burns damage the entire skin, that is, all of the dermis and epidermis. Any burn that damages below the skin is known as a 4th degree burn. I cracked up the first time I saw this.. and then laughed even harder when the mistake was repeated a couple episodes later! From what I've heard, this may have been repeated yet AGAIN in episodes I haven't seen..