In the film, Miriam Grant (Rosamund Pike) and the 2nd Mrs P (Minnie Driver) both attended McGill University. Ironically, out of all three actresses to play Barney's wives, it is the actress playing Clara (Rachelle Lefevre) who is an actual graduate of McGill University.
Paul Gross who plays Canadian Mountie Constable O'Malley of the North also played a Canadian Mountie, Constable Benton Fraser, on a popular TV series known as "Due South" [1994-1999] and having him play the Mountie in this film may be seen as an "in joke."
During Blair's discussion of the future of veganism over dinner he ponders whether one should say "in less than 50 years" or "in fewer than 50 years". He incorrectly settles on "fewer" reasoning that you're counting years and not measuring time. A ten-year calendar has fewer than 50 years on it, but something that will come to pass sooner than 50 years from now will do so in less than 50 years.
When Barney flips out because he cannot remember Miriam'a telephone number he grabs the bottle, which is obviously a break away prop, with such force that the bottle breaks while he is still lifting it upward.
In Rome many of the road signs are clearly from today. The two small format ones at the corner of Via Condotti and Piazza di Spagna are an example. Also the out of focus no parking signs on the doors of many buildings are clearly of the most recent kind.