Continuity mistake: Ted says that Marshall and he only ate jerky and drank Tantrum on their road trips. In 2-17 "Arrivederci, Fiero" however it's established that Marshall had a rule forbidding any kind of food or drink, even groceries, in his Fiero. (00:01:15)Tanngrisnir
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 explaining the curse of the blitz, they are all shown at the bar. But the blitz guy says he is going to leave because he just got Madden 2k1. If it's 2001, why is Robin in the shot? They didn't know her then.
Character mistake: Marshall's whole career focus is saving the environment, so it's a poorly written character choice that he buys plastic glasses to avoid a dish-washing feud, boasting that they can just throw them away. (00:20:10)
Continuity mistake: When Lily goes to the bar with her friend Michelle, Barney says "I want her around all the time." It then cuts from Barney to Lily's slightly shocked looking face, obviously from a previous cut/scene change, sitting at the table with them - she should still be walking to the bar with Michelle.