Sweet Taste of Liberty - S1-E3
Revealing mistake: Barney tells the cab driver to lock the doors, and he does. We then see Ted trying to open the door and you can hear the taxi is still driving by the engine noise. However, the scenery outside the windows has stopped moving. (00:01:50)
Revealing mistake: When Robin storms into her room with her gun, she slams the door and the entire wall shakes. (00:20:00)
Slapsgiving - S3-E9
Revealing mistake: When Marshall slaps Barney in Slapsgiving you can see that his hand doesn't actually touch Barney's face and there is no audio to match the slap.
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.
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.
Revealing 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.
Revealing mistake: In the big song and dance number, during the line "Girls will go and girls will come, but there's only one absolute", one of the dancers in the second line that runs in from the right is wearing trainers, not dress shoes.
Subway Wars - S6-E4
Revealing mistake: When Ted uses his laptop to get directions to the restaurant, Bing shows Los Angeles (where it's filmed) as their current location, instead of NY where the show is set. (00:02:15)
Revealing mistake: While showing the clip of Marshall streaking the first time, you can see thumbnails for videos of him streaking in New York. Those videos were created later that day and could not already be posted. One also shows a woman in yellow and appears to be a retake of the scene. (00:06:15)
Symphony of Illumination - S7-E12
Revealing mistake: While Marshall is on the roof putting up Christmas lights he accidentally drops his cell phone, which supposedly then falls to the ground where Scott picks it up and holds him hostage on the roof. But the cell phone can be seen to fall harmlessly into the gutter and can be seen behind the ladder, despite the added sound of it hitting the ground. (00:10:45)
Band or DJ? - S8-E13
Revealing mistake: When Jason Segal (Marshall) gets blasted in the face with confetti, you can see him squint his eyes in preparation for it.
Revealing mistake: When Lily brings out the "leaking" beanbag, you can see that she has the little white pellets in her clenched fist and is dropping them periodically, making it seem as if they're coming from the bag.
Revealing mistake: When Barney and Ted are holding hands on the beach and it starts raining, you can see that the sand is only wet in the place where they are sitting, but dry behind them, and on the left and right side too. (00:16:35)
Revealing mistake: When Ted is on top of The Lighthouse (with both girls), his and the girl's hair is blowing in the wind, but the trees behind them aren't moving at all.
Suggested correction: Depending on the location settings on his laptop, they could be giving a false reading. My laptop shows I'm in Cardiff Wales while in fact I'm nowhere near.
Ssiscool ★
The show was actually filmed on a film set in Los Angeles which is probably why the laptop says Los Angeles as the current location. This most probably slipped past the editing crew.