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.
First Time In New York - S2-E12
Continuity mistake: During Ted's second flashback to how he lost his virginity to Molly (the real story), as Molly gets out of bed you can see she's already wearing pants, but after we cut she stands up and starts putting them on.
The Chain Of Screaming - S3-E15
Audio problem: At the very beginning, after Ted says, "A new car", Lily's lips move, but she doesn't say anything. Obviously her dialogue was cut in post-production. (00:00:10)
Continuity mistake: Robin says she never played any team sports in 1-6 "Slutty Pumpkin". But in this episode she talks about her relationship with her father, being on a hockey team that won a championship and she made out with her teammate.
Life Among the Gorillas - S1-E17
Continuity mistake: At the karaoke bar, just before Lily storms off, the song book changes position on the table between shots.
Continuity mistake: In the car, just after Barney discovers they are not headed for Atlantic City, the DVD screen that has been hanging down from the car roof is suddenly stowed away between shots.
Continuity mistake: When Ted leaves the bar he leaves his phone behind and yet he somehow still makes a phone call to Lily from the subway.
Continuity mistake: In the opening scene at the bar, the level of beer in Ted's glass goes up and down between shots. (00:02:30)
Continuity mistake: When Marshall runs after Ted to tell him to get French Onion Dip, his robe keeps changing from crossed over his chest to straight down. (00:10:10)
Continuity mistake: When Ted and Tracey are on top of The Lighthouse, the trees behind them change variety between close ups and the shot zooming out.
Continuity mistake: When the Hula girl realises the penguin is Barney she holds her hands out and walks off. Camera cuts and her hands are on her hips. (00:17:30)
Continuity mistake: In this episode we get to see when Barney lost his virginity to Rhonda, The Man Maker. They had sex in her bed. But in season 2 (12th episode, First Time In New York), Barney says that his comforter smelled like menthol for a week.
Say Cheese - S5-E18
Continuity mistake: Barney claims in this episode he never "takes a bad picture". But in earlier episode "The Sexless Innkeeper" there were plenty of bad pictures of Barney in Marshall's Best Night Ever Montage.
Slapsgiving - S3-E9
Continuity mistake: Marshall turns on the computer, and a readout pops up clearly displaying 2h 45m left on the countdown. He then says "look at that - that means we're in the final hour left of the countdown".
The Playbook - S5-E8
Continuity mistake: In season 5 episode 8, Marshall and Ted are playing on an Xbox 360 Console. If you look carefully, you see that Ted's Xbox controller has no batteries in it, yet he is playing the game with Marshall, and then a few scenes change and so do the batteries. Scene 1, 12 minutes and 23 seconds: Marshall's remotes has a battery in while Ted's is out. Scene 2, 12m 40s: Marshall's Xbox control has no battery in. Scene 3, 12m 51s: Marshall's battery is back in the controller. Scene 4, 12m 58s: Marshall's battery is out again. Scene 5, 13m 44s: Marshall's battery is back in.
Continuity mistake: When Randy reveals that he didn't sleep with Robin, the blood from his nose switches nostrils between shots.
Continuity mistake: In the closing scene at the bar, the level of beer in Marshall's glass goes up and down without him drinking any. Also, the glass goes from in his hand to on the table in an instant between shots. (00:19:25)
Chosen answer: She was finally shown at the very end of the last episode of season 8, continuing into season 9. The actor's name is Cristin Milioti: http://www.imdb.com/name/nm2129662/.
Knever