Continuity: When Everett puts the ring on Julie's finger, the father (Kelly) is looking from behind Everett. When we see Kelly from the front his glasses are on top of his head. When we see him from the side, his glasses are on his face.
Errors in geography: Some scenes of the Family Stone's "New England town" were actually filmed in the Borough of Madison, NJ. No problem there - it could pass for a New England town. However, an aerial shot of the town square clearly shows a New Jersey Transit train pulling out of the station - definitely not something one would ever see in a New England town!
Continuity: When Ben is telling Meredith not to dilly dally, his shirt is outside of his pants. Then when he is talking to Everett it is tucked inside his pants.
Errors made by characters (possibly deliberate errors by the filmmakers): In the movie the character Amy Stone wasn't engaged. But in many scenes and best represented in the scene where Amy is opening the gift from Brad, the actress Rachel McAdams is seen having a ring on her left ring finger, possibly an engagement ring since her and Ryan Gosling were still together at the time.
Boom mic visible: Over Amy by the window while the family is gawking at Meredith outside when they first arrive.
Plot holes: Everett and Julie look for Meredith by visiting the inn and walking through town. During this time, the movie indicates their relationship is developing. Later that evening, when they've finished their searching, we see Everett dial his cell phone and Meredith's then ringing while she's sitting with Ben in his car. Dialing someone's cell phone is one of the first things you do when you're looking for them. In reality, Everett would have dialed her number at the outset of the search.