Other mistake: In the flashback where Gael is playing a guitar and singing a song, the chords on the guitar change, but his fingers do not move at all. He is playing the same chord the whole time.
Continuity mistake: When Ted, Barney and their dates are in the tub, Barney is holding a bottle. He takes a sip (i.e. no cap) and hands it to Amy (Ted's date). Now it has a cap. She hands it to Ted who unscrews it and has a drink.
Continuity mistake: In the scene where Ted goes over to Robin's apartment to yell about their breakup, Robin says "You shaved" and "I liked the beard," implying that this is the first time she has seen Ted since he shaved. But the night before she was in the kitchen at his apartment asking if he was okay with her dating Gael. At that point Ted had already shaved. (00:17:45)
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