At Universal Studios in Orlando, Florida, they used to have a ride called BACK TO THE FUTURE: THE RIDE (
http://www.imdb.com/title/tt0101392/). It was my favourite ride of the park, and I know alot about it, including a little secret that when they first opened the ride to a test public, that almost everyone would throw up on this ride. Quite a disaster for an $80M investment..!
The BTTF ride is a flight-simulator style motion control ride that makes people feel like their are flying by moving them in sync with the movie splashed on a giant IMAX screen.
What the ride makers realized was that there was a very slight discrepancy (in milliseconds!) between what the riders saw on screen, and what riders felt. This "lag" was enough to make people toss their cookies. Some adjustments in the software was all it took to fix the problem. With the visuals + movement in sync, the barfing stopped.
Some "3D" movies were not shot in 3D at all. Clash of the Titans was originally 2D and was re-rendered in 3D (I would love to know how that works). When Superman Returns (
http://www.imdb.com/title/tt0348150/) came out in 2006, most of the movie was 2D except for 20 minutes or so which was 3D. Using software, they took actual scenes filmed with a 2D camera (1 Lens), and converted them into 3D. It looked "ok" and was kind of cool, but it sucked too. The images were blurry, the flickering gave me a headache, and it was "no fun". At least one person in the audience was sick.
AVatar on the other hand was shot with a real 3D camera. It had 2 lens that were the same distance apart as your eyes. All of the CG scenes were rendered in 3D as well of course, but the "live" 3D was just as nice. No headaches at all, and definitely no one got sick that I noticed.