Whenever there is a shot of the street outside of the hotel, the same truck and cars drive by.
During the car chase, at first you can see the garage door opener on the driver's side visor, but it disappears when they go into the building, then reappears again.
Stewie is reading "Garfield at Large" (a comic book which chronicles every "Garfield" strip from June 19, 1978 to January 22, 1979) and mentions the character Nermel. But Nermel wasn't introduced to the strip until September 3, 1979, which is chronicled in Garfield's third book "Garfield Bigger Than Life."
When Peter steals Mel Gibson's movie, he just takes the canister and leaves the film on the projector.
Of all the shows that Fox had cancelled during the show's hiatus, Peter mentions Costello (1998); which was actually cancelled several months before Family Guy premiered. Costello premiered in September of 1998 and was cancelled by Fox a month later.