Old content showing with no caching. Weird

I have a weird problem.
I use WP 4.1.1 + Nginx + FastCGI. I have created a new post and when I checked what it looks liek for the end user, it was corrupted, the footer was not showing, etc.
Ever since, no matter what change I make to the post, it keeps sending the corrupted file to the browser.
Here is what I figured out so far:
1. It is not caused by any plugin. I have disabled all plugins and it behaves the same.
2. I run revision deletion and database optimization, and after it, the database contains only the latest version of the post with new content.
3. After i access the page in the browser, I can see that FastCGI cached the NEW content in the /var/run/nginx-cache/ folder. Yet Nginx spews out the old corrupted content
4. It is not caused by the browser cache. I use developer console in Chrome to verify that it is receiving fresh data from the server. It receives the same data in Incognito mode as well.
5. I have changed the permalink for the post, and the NEW content shows up. If I change it back to the original, or if I just access the original permalink, it sends the OLD content again.
6. I have even deleted the post, cleared the trash, and yet when I access the permalink, Nginx still spews out the old content.
7. I use OPCache but it cannot be caused by corrupted OPCace file as I rebooted the machine several times, meaning that OPCache in memory got wiped.

My conclusion is that the cached FastCGI file gets further processed somehow, just before being cached by OPCache, and then sent to the browser, and that the intermediary fil, in between the FastCGI cache and OPCache is somehow corrupted and overwrite fails, thus Nginx sends the old corrupted file.
I am at my wits end, how can I figure out where is the file I need to delete?

Thanks