The Clown Painting

Yellow Guy paints the head and shoulders of a cyan-haired happy-looking clown wearing a hat in the first episode, Don’t Hug me I’m Scared, saying "I might paint a picture of a clown". Before he can complete it, however, Sketchbook dumps oil on it in reprehension, saying "Whoa there, friend, you might need to slow down".

It went on to appear in subsequent episodes as a hidden easter egg, including Wakey Wakey.