It's times like these that I curse the fact that I get lazy when I read a good fic and don't save it! I'm hoping someone can help me out here, cos I'm going crazy!
It's a fic where Sam and Dean go to an abandoned carnival or funhouse or something, and something begins to take over Dean's form. He starts forgetting things, like which car is theirs, their hotel room etc and Sam finds that he's been writing down information on scraps of paper to remember everything...until eventually the something else is inside Dean and Sam has to go underground to find some kind of mirror to get Dean back? I think?
If anyone knows what I'm talking about with my garbled description I'll be eternally thankful!

Thanks - found! It's 'Receeding' by Minkmix - the link is in the comments cos I'm not that great at the whole linking thing...thanks again guys!