Continuity mistake: Josh and Amy are in a restaurant having breakfast and talking about the vote - the sugar dispenser can be seen next to some yellow flowers. In subsequent shots the flowers are still there but the dispenser has disappeared, even though no one touched it.
Character mistake: The President refers to Beowulf, saying that it was written in Middle English. He's mistaken; it was written in Old English, or Anglo-Saxon.
Chosen answer: A really annoying innate lack of sense. My friends and family say the same thing all the time, and I'm endlessly trying to correct them. I think people just don't know any better and (ironically) couldn't care less that they're speaking incorrectly.