I was tripping on acid several years ago and saw a huge jumping spider crawling up my leg. It's been years since I've used LSD, but that spider still unexpectedly visits me every now and then.
A Deka VU is actualy a error in your brain at the moment itself, that's why you think you experienced the situation before. So that's the reason you can't remember what the moment was. IT never happened
I've been doing something wrong then. Usually I know exactly when it happend.