Sweet Taste of Liberty - S1-E3
Revealing mistake: Barney tells the cab driver to lock the doors, and he does. We then see Ted trying to open the door and you can hear the taxi is still driving by the engine noise. However, the scenery outside the windows has stopped moving. (00:01:50)
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