Continuity mistake: When Mac's on the windshield and the car starts to dodge cars, one shot from inside the car shows the alien has disappeared.
Continuity mistake: When Garfield is in outer space asking the computer to tell him if he's really living in life number 9, his armchair vanishes for an entire shot. (00:40:00)
Visible crew/equipment: When they're building the petticoat balloon, the camera slowly goes back. Look at the lower-right corner of the screen: there is a man wearing a stadium jacket. He's a crew member.
Continuity mistake: When General Kael takes the baby to Nockmaar Castle, it has an extra 'turret' on the left-hand side, which has disappeared when Willow and Fin Raziel face the castle later on. (01:31:40 - 01:41:15)
Revealing mistake: When Jemma enters the headshrinker's laboratory, there is a table with three heads on it each encased in glass. When it goes in close-up on the middle head, you can see that the table has a hole where the actor can slip in his head.