Buffy: The Vampire Slayer - When Spike Gets Engaged He Doesn't Glow with Happiness, He Reflects with It
To find this slip-up wait until around the middle of the episode when Buffy and Spike get engaged. While in Gile's apartment, Buffy sits on Spike's lap in front of a glass wall sized cabinet. If you look at the cabinet you can clearly see Spike's reflection for a good 1-2 minutes.
Episode "Something Blue" and a television