I recently updated my multisite install to WordPress 3.5 and found an odd behavior when setting a featured image on an old post.
I was able to upload an image directly to the media library. WordPress saved the file in the current month's directory (...../blogs.dir/blog#/files/2013/01). When I edited the post and tried to set that newly uploaded file as the featured image, I got an error that the upload directory was not writable (...../blogs.dir/blog#/files/2011/10).
This situation makes me ask two questions:
1. Why would WordPress need to re-upload the file to set a featured image?
2. Why is it trying to write to an old directory that is based on the original publish date of that post (October 2011)?