As Xander crawls from his ice cream truck into his room, the the blue screen outside the truck's side window partially leaves the visible area of the window, revealing the studio. Then, the chroma key effect is turned off too early, showing the blue screen in its true color. This is only visible in the expanded-to-widescreen version, not in the originally intended 4:3 frame.
(at around 41 min) When Buffy and the first slayer roll down the hill fighting, you can see a camper van in the background. This is only visible in the expanded-to-widescreen version, not in the originally intended 4:3 frame.