One explanation I heard for the sensation of deja vu (and one that seems quite reasonable to me) is that it is the hindbrain's fault. The hindbrain is the part of your brain that stores stimuli that surpass a certain threshold (e.g. those associated with serious threats), in order to be able to quickly recognise similar situations, and react accordingly. Unfortunately, the information it stores is so "generic" that it is unable to distinguish a new situation with an old one, hence the feeling of deja vu. That said, it can still be quite useful, if for example the new situation poses a similar threat.
This morning, I had a deja vu of the more rational kind: in the back of the car, my kids were playing using a rubber band as an improvised guitar, in order to play along with the music from the car radio. They called out for me to have a look, and I almost did, even though I have a firm rule never to turn my head towards them while driving.
So where does the deja vu come in? I can hear you wondering. It is this: when I was three years old, I had a scooter that I would ride for hours (or at least that's how it seems now). But it did not have a bell with which I could warn people to get out of my way. So I improvised one by spanning a rubber band between two parts of the handlebar. It worked quite well ... up until the point when one end let go. I decided that there was no need to slow down; I could repair as I sailed along. Bad idea: while repairing my improvised bell, a picket fence suddenly decided to cross my path. (And I have still have the scar to prove it).
So in fact, my hindbrain should have warned me not to listen to rubber bands.
(Just in case you missed the reference, think of the crows in Dumbo singing "When I see an elephant fly").
