Marshall, Lily, and Ted went to Wesleyan, where Ted studied to be an architect, but Wesleyan does not have an architecture program.
In the Halloween episode in season 1, Robin says she never played any team sports. Just Tenis. However in a later season she mentions how her dad caught her kissing a hockey team mate.
When Lily is approached by the girl that's been sabotaging Barney while he's trying to pick up girls, the girl's hair is wavy/curly - and her voice definitely not Britney Spears. Later we find out that the girl who talked to Lily was Abby, Britney's character, and the scene is shown again - this time with the actual Britney in it, straight hair and all.
In Single Stamina (2.10), Lily and Marshall are married at Barney's brother's wedding, as evidenced by the rings on her finger, and Marshall's reference to her as his wife. At the scene, Ted and Robin dance, which indicates that they might be still together, even though during Something Blue (2.22), in which Marshall and Lily get married, Ted and Robin are broken up. However, the fact that Ted and Robin are not tired at the wedding (having Single Stamina) while Marshall and Lily are tired (having Couple's Coma) indicates that they are broken up, just on good enough terms to dance with each other.
Because during most of filming of season 3, Marshall had longer hair there are several flashback moments of Marshall in the time line of the first two seasons where he has long hair, but when the first 2 seasons actually took place he had shorter hair.
The time on the clock in the apartment is always 4:20, at least during season 1.
Ted is always shown at at drafting bench, drawing by hand, but most architects no longer do this. Instead, they use large screen computer displays with software such as AutoCAD.