Everybody makes everything up.

Okay, not everything maybe, but a lot.

False Memories of tragic and happy events - Boing Boing:

"...two articles on the fascinating subject of false memories. The first is a piece in yesterday's Guardian stating that four out of 10 people surveyed in a new study claim to have viewed footage that simply doesn't exist of the 7/7 bombings in London three years ago. A previous study by the same researchers, led by University of Portsmouth psychologist James Ost, reported how people distinctly remember seeing footage of the Princess Diana car crash. No such footage of that event exists either...

Another similar study from 2001 at the University of Washington addressed good false memories rather than bad ones. From Daily University Science News:

About one-third of the people who were exposed to a fake print ad describing a visit to Disneyland and how they met and shook hands with Bugs Bunny said later they remembered or knew the event happened to them.

The scenario described in the ad never occurred because Bugs Bunny is a Warner Bros. cartoon character and wouldn't be featured in any Walt Disney Co. property, according to University of Washington memory researchers Jacquie Pickrell and Elizabeth Loftus."