WARNING: This may give you seizures!
This is the original teaser trailer cut for William Friedkin's THE EXORCIST in 1973. As you'll see, it's quite a disorienting experience, and was never used (I don't think) by Warner Bros. The word is that it was deemed too terrifying for audiences, so the studio "banned" it; it's more likely that the strobe effect was just too irritating.
And boy, does this thing go on forever. Can you imagine being confronted with this in a theater? Forget the horrors of the film's content, this preview would have you begging for mercy.