When Marshall and Lily are looking for couples to hang out with, it shows them living in the apartment they bought but they didn't move in until much later.
Barney shows Ted the website Marshall made about the double date night without typing in the web address.
During the music montage of "All by Ourselves", when Barney and Robin are walking outside McLaren's, the wooden base of a clearly prop grass patch around a tree is visible protruding from the sidewalk.
During Marshall's online music video, the dates on the calendar are wrong. It goes from Sunday the 15th to Monday the 14th to Tuesday the 13th then to Wednesday the 14th. Sunday should have been the 11th, Monday the 12th, etc.
While Marshall's calendar does go "from Sunday the 15th to Monday the 14th" it also goes from Saturday the 10th to Sunday the 15th.
Robin mentions her "tenth grade" teacher. Tenth grade is what Americans normally say, and Robin is Canadian. She should have said her "grade ten" teacher.