Other mistake: In the episode 'Anne' Kenn tells Buffy that time in his hell dimension passes more quickly, that one day on Earth is the same as 100 years in his dimension. This means time goes roughly 36,000 times quicker. When Lily falls into the portal, it's at least 10 seconds until Buffy jumps in after her, but Lily is still just sitting there looking stunned. If times moves that much faster, 10 seconds would become over 4 days. Would Lily still have been just sitting there?
Other mistake: In the final scene, where Spike drives down the road while singing 'My Way' he turns the steering wheel significantly, but in the external shots the car is travelling down a straight road, with no bends. He is turning it sharply to the right as the camera cuts to outside, but the car continues straight ahead.
Other mistake: When we first see one of the Hellhounds at the clothes store that Cordelia works at, it jumps through the window, smashing the glass, and then attacks a customer who's wearing a tuxedo. But the Hellhound attacks the customer several seconds after it smashes through the window, giving the customer a chance to escape. But the customer just stands there unaware of the Hellhound, having not heard the window being smashed.