I've had this same non-render render experience, too. I notice that it occurs mostly when I'm rendering to a larger window than the current document window. That's a clue that the renderer is running out of memory. Hair (your beard) is REALLY consumptive of memory. When I try to render hair, I usually get commit charges up over 700Mb (on a 256Mb machine) and the available Ram often falls to a megabyte or less. When that happens you are slowed to a standstill and I think Firefly's equivalent of the P4 "not enough disk space" message is to just not produce any picture. This is a bug all right! The workaround is to try to scrimp down on memory and make sure you have lots of disk space available (and I mean LOTS) for the render. Both hair and cloth use extra disk space and even a gigabyte of empty drive space may not be enough. Still, it should error out and message you, not just render and present you with a blank window.