I've often thought about this and I think Hollywood does skew our views of history. It's a lot easier and more enjoyable to rent a film and watch it in 2 hours than to read a history book. So that's where we get our knowledge. And many of these films are great films, but not always accurate.
A couple of films come to mind. One is Braveheart, the story of William Wallace. I forget the true facts, but I remember something to the effect that one of the main characters would have only been a child at the time. I believe there were also more technical historical innacuracies, like the type of arrows used or some such thing. I thought Braveheart was a great movie. It won a Best Picture Oscar and Mel Gibson won a Best Director Oscar, so I'm not alone. As time passes, what is going to be remembered about William Wallace? People who have seen the movie are probably going to remember it long after they've forgotten any of the actual history - if they ever even knew the actual history.
Another TV series was Roots. I don't know how accurate it was, but it had a profound effect on black culture at the time. It underscores how powerful film can be. All of the information about the horrors of slavery had been available in various forms before Roots came to TV, but these things didn't spark an effect the way Roots did in a film format.
Another example is the knight. Our perceptions of a knight are drawn mostly from stories of fantasy and have very little to do with what knights were really like.
People should understand that movies are for entertainment's sake and are modified to make a more interesting tale. Characters are sometimes made up to be a composite of several people. Instead of introducing a whole group of friends, one close friend might be the composite of all these friends. Key events may be omitted and or changed around. I think a lot of people assume that the people making the movie did their research and tried to get these things right. I think anybody who goes to see a "historical" movie goes assuming they will learn something and they walk out thinking that what they just saw is generally true. They don't know which bits are true and which aren't, but they're going to walk away with that film implanted in them. Now they know who William Wallace was.
On the other hand, there's documentaries. These can also have innacurate facts, but are less subject to dramatic license.