112. Making your level so complex that you can only beat it if you can beat it.
Tautologies aside, I hate when there are 40 exits to a room and only one works. Maybe a hint? Blind guessing can basically double your death count through no lack of skill on your part.
113. USING ALL CAPS IN DIALOG BOXES OH MY GOD I'M RIGHT HERE YOU DON'T NEED TO YELL!!!!!