When Gwen is talking to her sister by the lake, we see them embrace before it cuts to a shot of their reflection in the lake. In the reflection Gwen has sunglasses on her head - which she wasn't wearing before that shot.
While everyone is performing "Santa Booze" Garhardt's bed covers jump from smooth to rumpled and his air tube disappears. The colour of the lines on the tube also changed from red to blue when its later put back on.
At the wedding reception, when Gwen and Jasper are dancing, they begin spinning in circles while holding hands. In most of the shots Gwen is holding onto the bow from the back of her dress. But in one shot it magically appears on the back of her dress again.
During the 'Santa Booze' skit, Daniel (a former doctor) corrects Oliver's pronunciation of the disease 'Falcon' suffers from by saying something that sounds like "Melagioma Nestocytroma", but there is no such disease.
The goof item below may give away important plot points.
When Andrea overdoses, Gwen is wearing a gray shirt, her hair is up and she has a double headband on. After this, several scenes take place showing Gwen "dealing" with the incident. When she tries to reach out to her sister and catches Eddie sleeping with another girl (seemingly days later), she's wearing the exact same outfit and hair style.