The Way We Almost Weren't - S2-E23
Revealing mistake: Closer to the end of the episode, when Lilly and Oliver are lying on the floor glued to the chairs, you can easily see black strips on their backs attaching them to the chairs.
Revealing mistake: In the opening credits, Beefy Bert has a different hair colour to what he has in the actual episodes. This was not included in Series 3.
Psy vs. Psy - S2-E3
Revealing mistake: Whenever you see Shawn and Gus outside it appears to be warm but if you look in the background people are wearing jackets and heavy coats, revealing that during filming it was winter and cold out.
Revealing mistake: When we first see the army of Starks outside Global Dynamics, it's quite obvious that only one of them is actually the actor that plays Stark; the others are merely people who look similar.
Revealing mistake: In an early episode Jack introduces Tracey to all the TGS writers, but giving mini bios and facts about them in rapid Succession. Seconds later we go to a flashback of Jack's assistant holding up pictures of random people as Jack names them. The problem though is you can see that it's the same picture being used every time.
Revealing mistake: After Betty leaves Jodie the voicemail, when Betty looks at Henry's Facebook page with the intention of unfriending him, she moves the cursor to the "remove friend" tab. Then in Betty's closeup we see that tab's reflections on both lenses of her eyeglasses, but the problem is the reflections are not backwards. (00:33:35)
Revealing mistake: When Moss runs into the door, all walls on the set shake. (00:06:25)