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: 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)
Continuity mistake: The morning after the second party, the items on the coffee table change instantly between shots. Several beer bottles and blue cups vanish, whilst a red cup and red bowl appear. (00:11:00)
Continuity mistake: At the third party when Lily sits Ted down to talk to him, Marshall stands behind Ted with a beer bottle in his hand. The position of Marshall's hands on Ted's back changes between shots. (00:14:40)
Continuity mistake: After Ted's Saturday night party (second of the three), when Ted, Lily and Marshall are sitting on the couch, numerous things change or suddenly appear behind them. Clothing is instantly on the back of the yellow chair and black chair, a wood chair appears, furniture moves, etc.
Continuity mistake: In the opening scene at the bar, when Ted is debating whether or not to ask out Robin, Marshall's beer is in his hand when the camera is on him, but on the table when the shot is from behind him. (00:03:00)
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: Barney tells Lily that she's rubbing his leg. When he does, a wine glass suddenly appears in Lily's hand. (00:03:20)
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)
Continuity mistake: The foam on top of the drinks Barney puts down at the start changes drastically between shots. (00:01:50)
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