Does anyone have an idea what is causing this?   Background:  The sheet is a clothified hi-res square.  I used choreographed group to "pull it back" on the bed.  

The spikes do not appear on the preview panel.  I've tried rendering in P4, Firefly with and without raytracing and futzed with other settings, but always get the same result.  This is a firefly rendering using default settings.

Could this have something to do with lighting?

Thanks for any advise you can offer.