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: During the scene back in college, when Marshall and Lily are listening to Dr. X's show, at the first shot of the radio, you can see some popcorn there already, although Marshall has not thrown any, yet. Even if you claim he's been throwing constantly, a few shots later they vanish, and reappear.
Continuity mistake: In this episode we see that Barney punched a hole in the wall next to the door because Ted broke apart a girl fight. It is supposed to be under the calendar on the right side of the door. It is also said that Ted and Robin were dating at this time. But earlier, in the episode "Wait For It..." (3-1) We see Ted going through his break up phase after Robin. One of the phases is painting the apartment, and there is no hole.