Continuity mistake: In the scene where Barney is headed home from work right before his wife leaves him for the athlete, the sun looks like it's morning sun. After about 30-45 seconds into the conversation with his wife why she is leaving, the sun changes to a late afternoon. It is very apparent on the mailbox. The shadows completely change direction in a matter of minutes.
Deliberate mistake: After the first scene where Hannah's ex-husband walks in without knocking and picks up the kids, Hannah is looking at some family pictures including her, her ex-husband, and the kids (supposedly taken when they were still married). But in the pictures they're all wearing the exact same clothes that they were in the previous scene. Could be a coincidence but that seems doubtful.
Revealing mistake: Vinnie places two coin-filled water bottles in the trunk of his car and closes the trunk. As he is walking around the car to get in, the back of the car suddenly sinks down, which by itself indicates a special effect, as springs don't behave that way. But in addition, the rear tires, which would appear flattened if under a heavy load, are completely unaffected by all that "weight."