Late 80's I met a dude who really wanted to go back to his Timothy Leary days with his colour work.
This was before photoshop actions ..

At the time I was working in a E6 C41 lab.... what we would do is load up his film on the racks , make sure all other client film was past the critical point and then wait for the lift action to raise the film completely out of the dev about half way and flick on the room lights... this was colour negative film and the effect was pretty cool. Because it was colour negative printing different effects was very easy.

We all have heard of cross process C41 in E6 chemicals.... one of our clients had a formula that had us pushing the development 4 stops which meant he would have a real under exposing situation... The effect was very surreal positive images, many tried to match his look but few were successful.. I believe the severe under expose and severe over dev and picking the right film was the trick.