When Dennis is about to have sex with Ted (J. Crew Guy) after his birthday party, he reaches for the beside light to turn it off, but the light goes off before the click sound from the switch. He then turns the light immediately back on, but this time the click sound and the light going on are simultaneous.
When Leslie is giving her speech after Patrick tells her and Anne that he won't donate his sperm, she puts her backpack on her shoulder. When we see her from the back she doesn't have it on. She then has to put her backpack on again.