The goof items below may give away important plot points.
When Natalie hugs Donny after she is back to normality (near the end of the movie), the hug position is very different when seen from each angle. Facing Donny their heads are apart, facing Natalie their heads are touching. Each position holds as the scene switches between angles.
While deciding to go stop the big wedding, Natalie says "shit" in her hotel room despite having established previously that she cannot say curse words in her PG-13 world. This is not a goof considering that "Isn't It Romantic" itself is rated PG-13, and the MPAA allowed her to say "shit" in this movie. That means that it's a word that can be said in a PG-13 world.