When Dawn visits Spike in his crypt, Spike clearly has a cigarette behind his ear, yet he reaches for a pack of cigarettes in his pocket. Seconds later, the cigarette behind his ear is gone.
When Tara is telling Willow not to do more teleportation spells, her mouth doesn't move when she says spells.
When then passenger train pulls into Sunnydale station, the station sign reads 683 miles to S F (San Francisco), and 682 miles to L A (Los Angeles). It is geographically impossible to be within California's borders and that far away from both cities.
Spike is considered to be 'as strong' as Buffy, regardless of the chip in his head. However, in the Magic Shop with Dawn, he has difficulty picking up the troll hammer. In the season finale, Buffy uses it with ease.