During this episode, Ted's kids are dressed differently than they are for the rest of the series. The couch and background are also different. (The implication during the series is that, the whole show is meant to be a story being told in one sitting.)
Ranjit's cab clearly pulls up to a space on the side of the road outside Robin's apartment, but in all the scenes in Ranjit's cab, the backdrop in the rear window shows his cab in the middle of the road.
When Marshall asks Lilly to Marry him, the foil is still sealed on the bottle of champagne as they fall to the floor. Later, when they get up from the floor, the foil is completely removed from the bottle.
When Ted goes to Robin's apartment to give her the blue french horn, Robin's kitchen appears to be on the right side of the room. However, in later seasons her kitchen is on the left side.
When Ted, Marshall, Lily, and Barney are watching Robin report on her show, she closes her story saying, "Reporting from Metro 1 News, back to you, Bill." Instead of saying Metro News 1, she says Metro 1 News, differing from the name she had previously called the show earlier in the episode.
S1E1: Ted is talking to a Lebanese girl (turns out to Carl's girlfriend) and he starts talking about the perfect wedding "...simple ceremony, we'll write our own vows, band, no DJ....".
S8E13: Ted is adamantly against having a band, his one mission is to convince Robin to use a DJ and not go with Lily's suggestion of a band.
In Barney's first scene, when he's getting a shave, the barber was holding and using the straight razor incorrectly. It's fairly useless to move the razor in the opposite direction of the blade edge but may have been done for safety reasons.