I read an article about a study int deja vu. It basically said it was a memory short circuit. Every day you brain loads everything that happened into short term memory. At some point during the day, usually while you sleep, your brain processes all these memories and decides what needs to go to long term or permanent memory, everything else is tossed to the side. So, whenever you do something your brain scans your long term memory to see if you have done this before. If you have it can provide useful details. If not it loads it into short term memory and the process starts all over.

The important thing is whenever you do something your brain scans long term memory to see if you have done this before. Deja vu can happen if your brain hiccups and for some reason starts loading whatever you are doing at the time into long term memory. So your brain is scanning long term memory as you are writing to long term memory. It makes what you are doing at that exact moment feel like a memory even though it isn’t. It’s also why you get that weird feeling because your brain knows something is not right and it’s trying to figure out what it is.