Although Don had met the gang previously on Guess Who's Knott Coming to Dinner? (1972), the gang and Don don't know each other. This is despite Scooby recognizing Homer Pipsqueak as Don Knotts.
When the black cat walks by in the fog (when Fred is with Velma), Daphne's voice gets mixed with Velma's and you hear Daphne's voice say "Bad luck" when it should've been Velma's