Buffy The Vampire Slayer (1997)

Out Of Mind, Out Of Sight - S1-E11

Revealing mistake: You get a pretty fast but noticeable glimpse at the stunt-woman's face when Harmony is pushed down the stairs, and it's obviously not the actress.

TedStixon

Welcome To The Hellmouth (1) - S1-E1

Revealing mistake: When Buffy sneaks into the locker room and checks out the dead body, you can see part of the outline around the prosthetic vampire bite on his neck (it isn't quite blended in properly), revealing it's an appliance.

TedStixon

A New Man - S4-E12

Revealing mistake: In the scene where Giles is dusting his book shelf he takes a book from the shelf and reads from it, you can see that the pages have nothing written on them. (00:07:30)

bic

Once More, With Feeling - S6-E7

Revealing mistake: When Buffy is singing in the Bronze with Anya and Tara as backup you can see Tara bump into the pillar and then dust herself off. (00:41:45)

Dead Things - S6-E13

Revealing mistake: In "Dead Things," after Buffy and Spike sleep together, when Spike asks, "Are we having a conversation?" and Buffy says, "What? No?" There is a side shot of her. If you look closely at her back (pausing helps), you can just barely see Buffy's skin colored tube top. Trust me, it's there. The time code is 1:22.

PurpleSharpie98795

Wild At Heart - S4-E6

Revealing mistake: In the scene when Oz stops the female wolf from attacking Willow, and they both change and Oz tears her neck out, if you look closely, you can see the actor's real skin at the neck.