The 2001 movie
- What Could Have Been: The film was originally pitched as a serious drama to Columbia and DreamWorks; but when Ivan Reitman came on board, he had different ideas.
- According to some design sketches, the final form of the alien's evolution would've been a much more humanoid form. However, seeing how difficult it how to make this a realistic threat, instead they went with the giant cell.
- Had Bill Murray signed on, this would have been Ghostbusters III.
- The mall that features in the third act was, in reality, a ghost mall abandoned for several years. The production team added store fronts and set dressing to make it look fully functional.
- According to Ivan Reitman on the commentary track, contrary to popular belief, the prominent feature of Head&Shoulders was not a case of product placement; P&G wasn't approached for placement, no money was paid for the appearance, and the bottles shown were acquired by the production.