Revealing mistake: When Lily walks into MacLaren's at the end, you can see her pregnant belly bump due to Alyson Hannigan being pregnant during filming. Lily was not pregnant at any time during this season. Even if she did try to cover it up with her coat, the belly bump is still visible. (00:20:25)
Casual PersonSuggested correction: It's passed off as her eating to many and getting bloated.
Intervention - S4-E4
Revealing mistake: In the flashback when Ted damages the ceiling beam with the flail, you can see that the damage is already there before Ted actually hits it.
KneverRevealing mistake: In the front shot of Ted in front of Stella's apartment, it is painfully obvious that he is in front of a green screen and not actually on a street.
Intervention - S4-E4
Revealing mistake: When the encyclopaedia bookshelf falls off the wall in the flashback, you can see the wall being pushed out, showing that it was being manipulated from the other side by crew.
