...most people suffer a mild (i.e. non-pathological) epileptic episode regularly (e.g. a hypnagogic jerk, the sudden "jolt" that frequently, but not always, occurs just prior to falling asleep), it is conjectured that a similar (mild) neurological aberration occurs in the experience of déjà vu, resulting in an erroneous sensation of memory.
This is very interesting to me because I've never realized that these two events are related or that my mild little jolts in bed were little epileptic episodes. I never really knew what they were. Nothing serious, just weird. Plus they don't happen that often. Neither does my déjà vu but that freaks me out every time.
The question remains...is it just a weird quirk of our brains or is there something else going on? Memory is pretty powerful so it's very conceivable that it can build these 'memories' for us. Just think about how vivid some dreams can be. I know mine are so vivid and realistic that I wonder if I'm peeking into parallel universes. I mean if not, my brain should be a movie director because my dreams are more entertaining that most of the drivel from Hollywood these days! Precognition seems unlikely to me as there are way too many variables to consider to look into the future.