The position of Spike's hands on the sword after the knight stabs through the roof of the Winnebago change with each shot, as does the length of sword the knight managed to get through.
At 19:38 you can clearly see Sarah Michelle Gellar's stunt double spinning in the fighting scene and not her, a couple of times.
When the knights of Byzantium march out off the woods, a crew-member in a red flannel shirt is visibly standing on the right side of the screen.