An event unfolds before your very eyes and you could have sworn that you've seen this exact event somewhere before...
Déjà vu has been heavily studied by various scientists for several decades. It's a very common experience for very normal people1. The mechanisms behind it are widely disagreed by scientists - ranging from a faulty recognition system2 to confusion of familiarity3, 4 to coming really close to recalling something else5. You can be blind and you can still experience it6 because sight is only one of the senses in contact with Polarity. With a good electric shock at the right place in the brain, you too can experience deja vu7. There has been a lot of intrigue without any agreement as to how or why it works.
Scientists have focused their efforts on what's known as stimulus-response testing8. Basically, it's a method to create an educated guess as to how something works by poking at it and seeing how it reacts. Think of poking a stick into an ant pile, watching ants pour out, and then trying to guess why they poured out. The end result is usually a theory - not fact.
8 Testing Response-Stimulus Equivalence Relations Using Differential Responses as a Sample. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1592360/