While Ted and Marshall are sitting at the bar, Ted reminds Marshall of how they met as roommates; "I got up to my room, and there was my roommate, Lying on the bed with his feet up on the wall...". Later on in the show, there is a different clip shown of how they met, with Marshall "eating a sandwich" and thinking Ted was the Dean of Students.
On Day 22, Ted says it's Sunday, and on Day 67, Marshall says it's Sunday. But if Day 22 was a Sunday, Day 67 would be a Wednesday.
Firearm laws in the State of New York are far stricter than depicted in this episode. New York considers the act of touching a handgun without first obtaining a concealed weapons permit to be a violent felony; Robin would be unlikely to have one given her Canadian citizenship, and Marshall outright admits he's never handled a firearm. Additionally, before the Bruen decision, New York was a "may-issue" state, with the City in particular rarely granting a carry permit to an applicant.
Robin claims to be very experienced with firearms, but comitts two major safety errors when she takes Marshall to the range. First, she points her loaded gun right at Marshall while they are talking. Second, she gives Marshall the gun while it's still loaded. Someone experienced with firearms would know not to point a loaded gun at a person they don't intend to shoot, and would not hand a loaded gun to anyone, let alone someone so as inexperienced with firearms as Marshall.
When Marshall pictures Lily having dinner with George Clinton, he gives her a ferret. Lily is supposed to be in San Francisco, CA. Ferrets are illegal in California. (Marshall hasn't ever lived in California, so he probably doesn't know the nuances of California laws.)