Most mysterious/unknown lego sets?

[Gregdabrat]

I created this post, to see what you guys think are the most unknown and undocumented sets. Like lego 4393-1.

[MAB]

Much of what bricklink counts as gear would probably be included, as some of it is a bit obscure. You could take a clock like that and add a Jack Stone figure to it...

 

[1974]

Yeah, gear should be excluded in this discussion as there is probably tons not on BL. I know I got a few myself

There's this set :

Which probably never existed

And this one, long belived to be a hoax :

This hospital from an offcial catalogue never got into production :

This one looks fully developed but never released :

Edited May 5, 2021 by 1974