When Richard Thomas was asked to do Hyalopterous Lemure's art (Ice Age), he mistakenly drew a "Lemur" (the fuzzy mammal) instead of a "Lemure" (ghost of an unburied dead). He added wings (Hyalopterous) and the result was a cute winged creature instead of a scary ghost. A reference to this mistake was later printed on Viscid Lemures's flavor text.

Source (Mistake #14)