Continuity mistake: Bartlet agrees to have a photo op with a goat and says "let's do it right now" - as C.J. turns to her assistant, we can see the Pres is already half way down the hall, even though this would be impossible in real time.
Continuity mistake: When Donna is in Josh's office, picking up folders, she puts a blue one on top of the pile; the next split-second shot from behind her shows a brown folder on top of the pile - then she places a blue one on top of that.
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.