One of the larger misconceptions about Christianity is, "why would I stop sinning if Jesus has already died for my sins?" Christians unfortunately have a poor track-record explaining the concept of "salvation apart from works" vs. "heavenly rewards based on our works."
Taking the Lord's name in vain. Christians the world over have been threatening their children and grandchildren with punishment if they so much as think about saying "Oh my God!" These Christians couldn't be more misguided.