I think I saw that before, but where?!!
DeJa Vu, you probably saw that movie as well. Quite a strange phenomenon. Let me show you an example. You are walking in the street and saw a person or rare looking tree. All the sudden you realize it's so familiar, it's like something you saw before BUT you know this is absolutely impossible because it's the first time you walk in this street.  The reason can get to very complicated details.
But basically i believe it's your left and right eye see one thing, but the left eye send send the signal to the brain faster than the right eye. So after you know the left eye saw something, then the right eye tells you again so you had like a SUDDEN realization of " oh, i think i saw this before, but i dont remember!?damn!!"
Just 0.01 sec delayed in sending message to the brain will cause that.

This phenomenon serve no purpose and not really that interesting to me.
I will show you the detail article on that if you like,,,

BUT DEja Vu is very similar to precognition. Precognition is like you walking in the street and you saw a bus stopped. A men got off, you realize he looks familiar, BUT somehow you know he's going to walk toward the right side. In this case you are able to predict something when DEJA VU can not. HOW and WHY? Most of the time people had this precognition in dreams. It happened to me a couple times but I was not able to remember the full details. I found this phenomenon quite interesting. I had a couple friends claimed they used to have that ability but now they don't have that anymore,,, could be many reasons but I will talk about it later,,