When Fez throws the first pickle slice at the Fatso Burger door, Donna turns and has no pickle slice stuck to her forehead. But in the close-up, the pickle slice is there.
When the gang is stealing Fatso the Clown, a car pulls up to place an order. Fez hides behind Fatso and takes the customer's order, the customer being Bob. After Fez/Fatso reads back the order, he tells Bob they're closed, which starts an argument between the two. As Bob drives off, the rest of the gang joins Fez, but the back of Bob's car is still pulling out of the drive-through, where he can obviously see Donna, Hyde, Randy and Fez.
When Randy and Fez ask Red to borrow the Vista Cruiser, the newspaper that Red is reading has a picture of a train in a paint scheme from 1982.
When Bob Pinciotti pulls into the drive-thru at Fatso Burger, the store is empty, the lights are off and the store is obviously closed.