More commonly known as the Scunthorpe problem, clbuttic is an issue that occurs with filters such as spam filters that mistakenly blocks or denies something that is valid. The Scunthorpe problem derives from the 1996 AOL swearword filter that blocked users living in Scunthorpe from creating accounts because of the obscene keyword in that town's name. Clbuttic is an issue that occurs with a search and replace technique that replaces swearwords with valid words. For example, replacing "ass" with "butt" in the word "classic" would make it "clbuttic".
Today, both of these terms are used to describe any similar problem that is encountered with software filters or search and replace regular expressions.