Continuity mistake: When Barney arrives at the apartment in his Top Gun outfit, we see him hang his sunglasses off his collar twice without removing them in between. (00:05:45)
Continuity mistake: When Barney is pointing out the woman in the blue shirt at the bar, Robin's drink changes from one hand to the other to the bar between shots.
Continuity mistake: In the opening scene at the apartment, the mug that Ted is holding keeps changing positions in his lap between shots. Sometimes it is tucked behind the leg nearest the camera, whilst in other shots it is sat on top of that leg.
Continuity mistake: When Lily and Robin are talking at the apartment, Lily's necklace keeps moving from on top of her jacket to hanging straight down over her dress.
Continuity mistake: When Marshall and Lily are talking in the jail cell they are holding hands, then the shot changes and both have their hands in their laps. (00:17:35)
Continuity mistake: When Barney is at the apartment in his Top Gun costume, the position his helmet is in on the coffee table changes between shots. Sometimes the front of the helmet faces the camera, at other times it is sideways. (00:06:00)
Continuity mistake: When Lily is making her list she takes the lid off the pen twice. (00:09:00)
Continuity mistake: When Ted and Marshall are watching Sandy Rivers read the paper on TV, the items besides the TV on the TV table switch according to the angle.
Continuity mistake: At the apartment when Marshall gives Lily the red cocktail, the green paper umbrella moves from facing away from Lily to facing towards her between shots.
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: In the first scene the karaoke bar, Barney's drink moves between shots from in his hand at his mouth to set on the table. (00:07:40)
Nothing Good Happens After 2 A.M. - S1-E18
Continuity mistake: In the classroom scene, the girl with the black dress with green sleeves keeps changing position between shots. Her arms are sometimes folded, sometimes flat on the table, and sometimes she is leaning on her elbows. (00:01:40)
Continuity mistake: When Lily and Marshall are discussing which band to book for their wedding, the way Lily's shirt hangs changes frequently between shots. Sometimes the shirt hangs straight, in other shots it is folded back on itself.
Continuity mistake: At dinner, Robin starts eating the ice cream sundae and knocks the cherry off the top. In the next scene, when she passes the sundae to Mike, the cherry is back neatly on top and the sundae looks untouched. (00:12:50 - 00:13:30)
Continuity mistake: The morning after The Wedding, when Lily is talking about throwing the bouquet, the mug in Marshall's hand changes position, vanishes and then reappears instantly between shots. (00:05:20)
Continuity mistake: At the bar, when Robin is telling the gang about Mike, Marshall's beer moves from in his hand to on the table between shots. Also, the amount of beer in the glass changes frequently depending on the camera angle.
Continuity mistake: In the scene when Lily yells at Robin, because Lily thinks her wedding is ruined, her hair changes. When Lily screams "Of Course I'm not okay." there's only one stray of hair hanging over her left shoulder, but a few seconds later when she screams "Oh grow up Robin." much more hair is hanging over her shoulder. (00:08:15)
Moving Day - S2-E18
Continuity mistake: In the final scene when Barney is reading his top ten list, the piece of paper moves from his left to right hand and back in an instant between shots,.
Continuity mistake: When Marshall and Lily approach the reception desk at The Wedding chapel, Marshall's cocktail moves from his right to his left hand instantly between shots. (00:07:55)
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