It happens when you see something and your brain tries to interpret it is 'just seen', but it accidentily goes to memory. Then the brain is like "Hey, I know that". and then you get the idea you saw it before.
You're brain keeps nearly every new piece of information it see's. Sometimes when you have done/see something you have once done in the last few days. You're brain automatically see's it and recognizes. It happens so fast, you get Deja vu.
Or it's because you have a future self of you running around doing things a few days ahead of you.
:v
[SIGPIC][/SIGPIC] <Jarmund>puta keppario Cogito ergo sum Ex-Tori Agent | OBT Member | [Reaper] | A.k.a. void/prada
And as far as I know it occurs mostly if you get too less sleep, because the brain is more likelyx to make a misstake then. (At least for me, it is true )