From Eterna Wiki
An "Illusion", also called Optical illusion, is a sequence that contains almost exclusively GU pairs. Like Cub scouts and Christmas trees, they are undesirable in labs. Some puzzles, however, add an element of difficulty by requiring solutions to have a large percentage of GU pairs.
Teacing about Illusions
RNA Lab: "Christmas Trees," "Cub Scout Projects," and "Optical Illusions" by Dimension9