Factual error: When Ben goes to Anna's yoga class, we see the very end of the class. Everyone is seated upright, cross-legged. In real life, yoga classes always end in savasanah, a pose where you lie on your back and breathe to take in all that the body/mind just did. They just had the actors seated upright to show them making eyes at each other.
Character mistake: About 3/4 of the way through the movie, when Jennifer Aniston is talking to Ben Affleck, she calls him Ben instead of his character's name Neil.