Continuity mistake: When Ted is giving up with Marshall's best man speech he is holding a pen in his hand as well as a beer bottle. In the next shot the pen disappears and he is just holding a beer bottle. In the next shot the beer is on the coffee table and Ted drops the pen onto the coffee table.
Continuity mistake: In this episode we learn that Barney doesn't know how to drive. But throughout the whole series there are reference that he actually does, such as: In 'Come On' (22th episode 1st season) : Penelope says to Barney: "We had sex twice in your car." In 'Moving Day' (18th episode 2nd season) : Barney drives away with Ted's rental van. In 'The Bracket' (14th episode 3rd season) : Barney stole Holly's SUV as they were camping.
Other mistake: When Ted and Robin borrow Marshall's Fiero and are sitting in the car talking as Ted drives, you can hear the beat of a song playing in the car that is not the Proclaimers' I'm Gonna Be (500 Miles), which in many future episodes is referenced as the only song that can be played in that car due to the tape being stuck in the tape player.
Continuity mistake: At the airport, Lily and Robin are talking off to the side away from others. During the scene, the cameras shoot from in front of the two actors and from behind Lily. From the frontal view, Lily's two headbands are white and gold-jeweled. The white headband is closest to her forehead. When the camera shot switches from front to behind, the headbands are switched on her head. The gold one is now closest to her forehead and the white one is behind.
Continuity mistake: When Robin is talking to her sister Katie in the Empire State Building lobby, Katie's bag moves between shots. Sometimes she wears it with the strap hanging straight down from her shoulder, in other shots the strap goes across her chest like a messenger bag.
Continuity mistake: During the scene in "Marshall and Lily's part", after they leave the bathroom, we can see Robin and Virginia (Ted's Mom) talking in the back. Their hand motions do not match the later scene of the "Robin and Ted's part". Obviously, they were shot separately.
Continuity mistake: When Marshall and Ted are on the phone to one another, the string from Ted's hoodie changes position between shots, much of this may be because he is moving his hand due to being on the phone. However the right string goes back and forth from being caught on the collar of his jacket to hanging straight down over his jacket just before he is on the phone to his mother.
Continuity mistake: In the scene when Lily yells at Robin, because Lily thinks her wedding is ruined, her hair changes. When Lily screams "Of course I'm not okay." there's only one stray of hair hanging over her left shoulder, but a few seconds later when she screams "Oh grow up Robin." much more hair is hanging over her shoulder.