I watched a short clip from Men In Black tonight with Tommy Lee Jones describing the relative intelligence of a single person and the relative stupidity of humanity as a whole. In the clip, he mentions the transition of the prevailing view of the universe from the Ptolemaic to a Copernican as an example of widely held knowledge that has changed. This is a good point, since the Copernican philosophy (that we are not special) has been upheld by virtually every major discovery in the cosmos since.
Then, he mentions the Columbus flat earth myth. In schools in the United States (and elsewhere?), we're taught that Columbus went to the Italians and the Portuguese with a plan to travel west to India, and they rejected his idea as preposterous because they believed the Earth was flat. What they actually believed was that Columbus' estimates for the size of the Earth were completely off base, and that the journey West was too far to manage without hope for a resupply. When Columbus finally set sail, he did so with supplies that would not have lasted him a hypothetical Spain to India voyage were the American continents not to exist.
I also watched some kind of advert recently for a movie where someone develops a pill that will help them "use the full potential of their brain." It trots out the same old "human beings only use (some low percentage, usually 10)% of their brain" myth, which is laughably false; anyone subscribing to a Copernican philosophy wouldn't allot himself an extra 90% of "untapped" potential, and if you gave it a moments thought, you could probably deduce a lot of the evidence that this myth is bunk yourself.
There are plenty of other commonly held misconceptions in all sorts of areas of study. Two from this list I see often in the popular media are the "You lose most of your body heat from your head" misconception, which persists confusedly even in people who have passed thermodynamics but persists as a way for mothers to get their children to wear hats, and the somewhat off base meme "sixth sense", which ignores a lot of interesting senses we possess, my favorite of which is Proprioception, which is perhaps a stumbling block for virtual reality systems in the future.
These types of things instantly distract from the point or concept being conveyed, even when they aren't particularly important to them. Tommy Lee Jones' Copernican example was a profound transition; he didn't need the false 1500's flat earth myth. It reminds me of watching old popular science or medical videos, where housewives wash living rooms down with a hose, preparations for a mars space journey include diplomatic concerns with the local intelligent Martian life, and where wishful narrators extol the limitless virtues of "the atom", with radiation hailed as a panacea. All at once, it makes me cognizant of how future generations will view our artistic and scientific forecasts as quaint, simplistic, and wrong.