What's supposed to happen to SharePoint when the IIS blob cache is full?

Can someone possibly point me at a description of what's supposed to happen to SharePoint if the IIS blob cache directory fills up?

I've seen postings from MS saying that you need to keep the directory 20% bigger than the amount of files you might end up storing in there, but that's clearly not the whole answer.

I ask because we've had a problem today on SP2010 with files disappearing from people's sites as soon as they are uploaded, or being replaced with files of 0 bytes.   It seems to be caused by the blob cache directory being full, and SharePoint not making any attempt to free up space in it.

Rob Schifreen SharePoint 2010 Admin University of Brighton, UK