Continuity mistake: During the scene back in college, when Marshall and Lily are listening to Dr. X's show, at the first shot of the radio, you can see some popcorn there already, although Marshall has not thrown any, yet. Even if you claim he's been throwing constantly, a few shots later they vanish, and reappear.
Other mistake: At the start of the episode it can be seen that the DVD player only has a power cord connected. Shortly afterwards only one cable (power) is seen connected to the TV set, then a few seconds later Ted plays a DVD.
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