Heard of Christmas in July? Well, it's Halloween in December!
In the Be Cool, Scooby-Doo! episode "Halloween," at 5:05 in the episode when Fred is unmasking the innocent kid dressed as Abraham Lincoln, Shaggy can be seen for a brief second dressed as the ghost of Mr. Hyde from "Nowhere to Hyde" (in Scooby-Doo, Where Are You?)
Thank you to Samuel Kirkwood for pointing this out!