Continuity mistake: When Lily and Marshall are talking in the kitchen, we see Lily put down the recipe card to the left of the jar of mayonnaise. The shot changes and the card is now to the right of the jar. (00:06:25)
Continuity mistake: When Lily is making the layered salad, in some shots the layers are defined, but in others there is much more mayonnaise running between the layers. (00:07:30)
Continuity mistake: When Lily drops the potato chips on top of the 'salad', we can see a few chips sticking up beyond the top of the bowl. The shot changes and now the chips are laid flat, with none sticking up over the top. (00:07:40)
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: 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.
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