I read somewhere once that one theory behind deja vu is that it's the result of a temporary de-sync between your left and right brain hemispheres. Usually signals reach each hemisphere at about the same time, but sometimes the signals can be de-synced by a few milliseconds and (theoretically) cause you to process the same thing twice in quick succession, giving you the vague impression of familiarity on the second signal process.

Edit: Messed up a bit. Each hemisphere sends a signal to the temporal lobe for processing, and that's where the de-sync can happen.