Character mistake: During the face-off about religion/politics, they get into a shouting match about commandments, both groups being equally convinced that "honour your father and mother" is the 1st or 3rd commandment, respectively. In actual fact it's the 4th or 5th, depending on which version (Catholic or Protestant) of the 10 commandments you go by. See http://www.positiveatheism.org/crt/whichcom.htm.
Character mistake: Sam makes a joke about Laurie being "free of cataracts" because she smokes so much pot. She laughs and says "I get that. That's funny." Except there's nothing to 'get', because Sam is wrong. Cannabis has been used as pain relief for people suffering from glaucoma; it does nothing whatsoever for cataracts. It's surprising that neither Sam nor Laurie would have known this. (00:03:10)
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.