Revealing mistake: While showing the clip of Marshall streaking the first time, you can see thumbnails for videos of him streaking in New York. Those videos were created later that day and could not already be posted. One also shows a woman in yellow and appears to be a retake of the scene. (00:06:15)
Symphony of Illumination - S7-E12
Revealing mistake: While Marshall is on the roof putting up Christmas lights he accidentally drops his cell phone, which supposedly then falls to the ground where Scott picks it up and holds him hostage on the roof. But the cell phone can be seen to fall harmlessly into the gutter and can be seen behind the ladder, despite the added sound of it hitting the ground. (00:10:45)
Good Crazy - S7-E22
Continuity mistake: When Ted is on a date with the 2nd girl you can see her tattoos but when Ted sees Robin the tattoos change. (00:09:50)
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