If you have a large queue of files set for download about half way through, I don't have the exact number it requires for this happen, the files will continue to "download" but cannot be found on disk.
I've seen this happen two different times now after upgrading to 6.7. The first time I set up a large queue and walked away. When I returned I saw that the files had supposedly downloaded without failure, but I couldn't find half of them. I have a very simple setup for download and extraction path (E:\DOWNLOAD\$(NZBFILE)\).
I watched this actively occurring with a very large set of files, they were showing as downloading, there is traffic on the NIC, but there is absolutely nothing being written to disk....at least that I can find. The logs do not seem to reflect this activity, but the size column under downloading files is definitely growing.