When Shaggy and Scooby fall into the tunnel, the background briefly shows the outside - with the sidewalk and night sky - even though the gang is still underground.
The out of order phone booth hiding the secret tunnel didn't have a phone in it.
When Fred is instructing Shaggy to pull the lever, which opens the cell Fred and the girls are trapped in, his arm seems to go through Daphne.
Just about all of the Puerto Rican characters have Mexican accents.