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.
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.