I'm having a bit of trouble replicating this error. Here's the steps I'm using to test:
1. Browse to (http://jqueryui.com/download/).
2. Uncheck selectable interaction.
3. Hit deselect all for effects.
4. Attempt to download distribution and verify that it works.
5. Uncheck droppable interaction.
6. Attempt to download again.
The first time I got to the last step, it paused for about three seconds and then allowed me to download a valid distribution. I think this combination hadn't been cached before and so it was attempting to build the distribution and cache it before allowing me to download it. Now, whenever I run this test procedure, it returns me the cached distribution immediately.
Not sure why the download builder would fail to cache this combination every time DarwinSurvivor tried to download it. It's possible that the user tried to cache under a peak load, although I'm not convinced it would fail repeatedly like it seemed to.
Was anyone else able to replicate this?