Continuity mistake: Lily and Marshall mention that they've been using the same toothbrush ever since they've been together, but in the 14th episode of the 1st season, "Zip, zip, zip", Marshall and Lily are simultaneously brushing their teeth with a tooth brush each (before Ted and Victoria get home).
Continuity mistake: In Ted's flashback where Barney's mysterious saboteur is talking to Lily, the girl has straight blonde hair. In episode 14, "The Bracket" where his saboteur was first introduced and she originally spoke to Lily, she had curly blonde hair in the same sequence.
Continuity mistake: In this episode, both Ted and Marshall mention that Lucy, Stella's daughter, is 7 years old, when in the previous season (13th episode of 3rd season, Ten Sessions) Stella states: "I have a daughter. Her name is Lucy, she's 8." Did she grow younger?
Continuity mistake: After the gang gets to the bar around the end of the episode, Ted starts his speech about how they all need to move on. As he moves over to the booth, he tells the people sitting there to get out. You can see that the girl takes her phone with her, and there is no other phone anywhere on the table. A few seconds later, she comes back saying that she left her phone, and now the phone is visible on the table.
Continuity mistake: Barney drives fast to prove to his friends that he can talk his way out of a ticket, however in the earlier seasons, when Marshall's car broke down, they tell a story about Barney driving and he cannot drive, and seems to have a fear of driving.
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.