Continuity mistake: Donna and the gang are in a restaurant/bar in New Hampshire and she has her hand up by her head as she eats. As Josh asks "are you eating the rest of the sandwich?" her hand is suddenly at her mouth. (00:32:10)
Continuity mistake: Pres. Bartlet is at his farm in New Hampshire, talking to Leo. In one shot he has an arm on the rail fence and the other hand on his hip. The next shot shows him with both arms on the rail fence even though he had no time to shift position. (00:41:25)
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.