If a subdirectory in a _posts directory is considered to be a valid post name, Jekyll attempts to construct a post from the directory. This stops site generation.
The problem is on line ~162 of site.rb where in read_posts(), it is only checked whether the directory entry is a valid post name. It should also check that the entry corresponds to a file, not a directory.
Ignore directories with valid post names.
Merged #875 which fixes this.