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.
Revealing mistake: When Lily walks into MacLaren's at the end, you can see her pregnant belly bump due to Alyson Hannigan being pregnant during filming. Lily was not pregnant at any time during this season. Even if she did try to cover it up with her coat, the belly bump is still visible.
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.