In the episode "Happily ever after" (S4E6) Robin talks about how she used to play ice hockey back in Canada. However in season 1 (the "slutty pumpkin"-episode) she claims she never played any team sports, only tennis.
In all the close shots of the group under the table after the almonds were delivered, the almonds are visible. However, in the wide shot of the group getting out from under the table, where we can see under the table and the rest of the restaurant, there are no almonds on the floor.
When the gang is taking a ride in the Taxi, Ted breaks off the window crank handle. But, no Crown Victoria vehicles of that generation have manual windows.
Four of the group sit in the back seat of the taxi (all but Ted). Only three people are permitted in the back seat of a New York City cab.
Barney mentions he used to visit a girl in prison. During the flashback two girls in prison start fighting and Barney takes out his phone to take a video. The phone he takes out is an Iphone 3G (which was not released until the year the episode aired).