While Ted explains his most embarrassing story, "The Re-Return," there are references and 'flashbacks' to the pilot episode. Ted is explaining that later that night after he told Robin that he loved her, there was a lot of drinking and Ted 're-returned' to Robin's apartment while drunk. Obviously the 're-return' was shot specifically for this episode. However, there was one major and several minor mistakes made during the 'flashback' scenes. The simple things were about how crowded the bar was, the lights and neon signs of the bar, the objects on the gang's table, and the tie choice that Ted wore. The major, and very obvious and easily-fixable mistake was Ranjit's shirt. In the pilot, Ranjit wore a cornflower blue jacket. Then, for the re-shoots, he was wearing a deep red long-sleeve polo shirt.
At about 1:45 while at MacLarens Lily asks about the re-return; at this point Ted is talking and you can see a guy with a light brown jacket sitting alone at the bar. When the camera turns to Lily then back to Ted there is now another guy sitting next to him then the camera goes back to Lily for a quick second then back to Ted again and now that second guy is gone.
The TV screen only has a power cord in it, no leads from the video player.
When Marshall is recalling the embarrassing time a student walked in on him in the shared bathroom at Lily's school, a quick shot shows that, while his lower back is somewhat exposed, his pants are around his waste, despite Marshall concluding the story, saying "I wish I'd pulled up my pants."
At the Bar, Lily leaves the booth to go talk to Robyn at the bar, using the fourth wall escape route. The booth has always been against the wall entered from the far end.
In the flashback when Barney is working in the coffee shop, Greg asks for a Latte but Barney gives him a Black Coffee.
Lily says the word 'what' which goes unpunished by Marshall, when she says "what was her name".