When Gus is eating the pineapple Peking dumplings, he opens the takeout box to eat. When the camera switches to behind him, the box is closed.
When O'Hara approaches Penny with the cupcake, there is a book open on Penny's desk. As they talk Penny starts to stand up and shuts the book. About a second later she starts to sit down again and the book is suddenly open again, exactly where she left it.
When Shawn returns to the set with the nail gun, we see a man changing the cue cards. The next card to come up is from the beginning of the scene, reading "...de caridad de Serena. Solo tengo que ponerme mi traje. Corrine: Dime, Chad, me veo..." - "...Serena's charity [ball]. I just have to put on my suit. Corrine: Tell me, Chad, do I look..."
After Shawn is dumped into the well at the end, you can see his hand under the water holding him down, but in the next scene you don't see anything under water.