Continuity mistake: In the 7th episode of Season 5, the scenes when the "Storm-Trooper" (actually, a robot) passes by The Window do not match. The first time we see it, from the outside, as soon as he passes, Robin notices him and points towards him, but the second time, from the inside, she doesn't point at him.
Continuity mistake: The scenes before Robin and Barney leave the diner vary depending on the angle. From outside we see that Robin fixes Barney's suit before they leave, but from the inside she doesn't.
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