Continuity mistake: When Buffy is out shopping for her dress and Pike is on his bike beside her, they stop to talk and have an argument. Buffy walks off and Pike turns his bike around to drive off, but as he does this, his bike suddenly moves a few feet away from the wall he was right next to in the previous shot. (01:01:05)
Continuity mistake: Towards the end of the film when they are getting ready for a dance, a Pepsi can mysteriously moves around on the floor in front of Buffy's friend. (00:57:05 - 00:58:00)
Continuity mistake: When the vampire is on the hood of Pike's car, his head is just barely on the windshield. However, in the next shot from Pike's view, the vamp's head is covering the entire windshield. This happens several times. (00:39:35)
Audio problem: When Buffy is using the flag as a weapon to fight Lothos, you can hear the metal clang of the sword, but the flag pole is made from wood.
Visible crew/equipment: After the opening titles, Buffy and her friends are shown at the mall in an overhead shot and the boom mic casts a shadow on the floor.
Continuity mistake: After Pike has crashed his van and he and Buffy are fighting the vampires, Pike gets body slammed by a vampire that's wearing a black hat. The vampire straightens up to face Buffy and the hat is suddenly gone. (00:41:55)
Answer: I'd say you treat them as 2 separate things. My personal opinion is that you should just watch the series and forget about the movie.