Déjà vu is one of the oddest feelings ever. Most of the time for me, I feel like I have been in the same situation in a dream before. Sometimes I randomly duck, hoping an object will fly over my head and I will save my own life, as well as looking awesome in the process.
I am not going to try to explain what causes it, but my personal theory is that I traveled back in time to that moment. We feel as if we have experienced a certain moment before because we actually have. In the alternate universe, I made some terrible mistake, having to retrace my steps to before the incident. Of course this usually involves some heroic situation of the world on the verge of destruction, as I desperately push the button to send me back in time as lasers shoot everywhere and explosions come from all angles.
But I usually just stop for a moment, then continue on with my day.
Chocolate Frosted Sugar Bombs 