Luce is standing in front a flower wreath. When Coop asks her to dance at the reception, he stands at her right side. In the next shot Coop is on her left side, the flowers disappear, and Rachel and Heck are dancing behind them.
At the wedding reception, when Rachel gets up to help Heck give a speech, Rachel had her right arm wrapped around Hecks left arm. The camera takes a shot from behind and their arms are separated. The camera then quickly goes to a frontal view and their arms are interlocked again.
When Rachel is in the video rental shop, she picks up "Georgie's Bush" from the adult section. When Rachel gives the DVD to the clerk, the cover is a woman wrapped in the American flag. When Rachel's mom comes into the video shop, Rachel has the DVD in her hand but the cover has changed to a woman wearing a boa who is standing over a woman laying down.
The goof items below may give away important plot points.
When Rachel and Luce make out in Flowered Up, Luce's hair is straight. When Heck arrives and Luce comes out of the back, she still has straight hair. After Heck leaves, Luce realizes that Rachel ran away out of the window, then she follows her. When she reaches Rachel, Luce has curly hair.
Before Rachel and parents head off to chase Luce. H is left at the parents house. Heck leaves. In the following scenes, H is shown comforting Heck at Heck & Rachel's apartment, which would be impossible.