Hahahah! I just remembered the perfect example for this. Near the end of season 5 of the TV series Scrubs, the main character J.D. meets Dr. Kim Briggs. She has supposedly been at Sacred Heart Hospital (the series' main setting) since before the show even began (back in 2001), but she makes her first appearance in this episode. Turk (J.D.'s best friend) points out to J.D. that even though Dr. Briggs has never been seen or mentioned up until this point in the series, she has been at the hospital all along. The episodes shows several flashbacks to previous episodes, and Dr. Briggs is superimposed into old snippets of episodes from the past (as a joke, of course).