It’s already been reported and a fix was committed but the bug is totally random and I can’t just sit in front of my computer all day for a week randomly adjusting nodes trying to see if the fix is good so this thread is meant to supplement the official bug report here. So that’s what this thread is about, more eyes on this is better.
This may have just been a problem with OSX so I’m not sure about Linux and Windows.
The 2.71 downloads should be fine so long as the commit solved this. For me, it was only RC1 and RC2 but I have not had time to check the newest builds.
If Cycles crashes while you have the Viewport active, please make a note of it here.
Here is the official tracker report. https://developer.blender.org/T40763
EDIT:Atom brings up a good point, this may have only affected CPU rendering, not GPU.