There are quite a few bloopers in buffy.

In "the pack" nearly all the supposed video clips of hyenas Willow watches on her computer monitor throughout the episode are actually of African wild dogs.

In the Season 6 episode where Buffy celebrates her birthday and Dawn indirectly wishes everyone remain in the house, Spike follows Buffy into the hallway right by her front door next to which there is a mirror. In this mirror you see Spike's reflection. Vampires do not have reflections, so Spike should not have been visible.