Revealing mistake: When Barney and Ted are holding hands on the beach and it starts raining, you can see that the sand is only wet in the place where they are sitting, but dry behind them, and on the left and right side too. (00:16:35)
Continuity mistake: When Ted and Tracey are on top of The Lighthouse, the trees behind them change variety between close ups and the shot zooming out.
Revealing mistake: When Ted is on top of The Lighthouse (with both girls), his and the girl's hair is blowing in the wind, but the trees behind them aren't moving at all.
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