Continuity mistake: When breaking up with Natalie, her hands are constantly changing position between instant cuts. (00:18:30)
Continuity mistake: When Ted mentions having a party, Marshall's hand goes from on his beer to on his chin instantly. (00:03:10)
Continuity mistake: In the pilot episode we see the Son and Daughter listening to the start of the story on how Ted met their mother. In the final episode of the show it's revealed that the whole story has taken place in one sitting. However, the clothes the kids are wearing and the items around them are different in the pilot episode to every other episode. The sofa is a different design, the cushions appear etc. (00:00:45)
Okay Awesome - S1-E5
Continuity mistake: Ted asks if he had a stroke at the start of the episode and puts his hand to his chest. Camera cuts and his hand has swapped from his right to his left on his chest. (00:00:35)
Continuity mistake: Barney tells Lily that she's rubbing his leg. When he does, a wine glass suddenly appears in Lily's hand. (00:03:20)
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)
Continuity mistake: When Barney is scoping the apartment, his hands are next to his temples. As the shot changes to a wider shot, they are now much further away from his temple. (00:08:05)
Continuity mistake: When Barney is explaining about there always being 1 girl who doesn't know who's party it is, his hand is sudden;y outstretched or by his side depending on the shot. (00:08:00)
The Best Burger in New York - S4-E2
Continuity mistake: When Ted picks up his burger it already has a bite taken out of it, then in the next shot the burger is whole again. (00:20:09)
Continuity mistake: After telling the joke about the Teenage Mutant Ninja Turtles, Marshall's hands change between shots, from holding his hand for a hi-five to his pockets to holding the Skittles. (00:08:32)
Continuity mistake: The foam on top of the drinks Barney puts down at the start changes drastically between shots. (00:01:50)
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.
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