Been messing around with cutting edge from the PPA. Which at this time has v2.78.4 on the header.
Not sure what’s causing it, but I’m getting some random black pixels which seem to be the negative equivalent to fireflies. I mentioned this happening before on occasion with the cutting edge version, and it was suggested it was a bug. In this case I’m using multiscatter GGX in the glass shader of the material, and the new light sample threshold setting. (Although there may be other factors involved.) So here’s the pasteall file if anyone wants to look into it:
http://pasteall.org/blend/index.php?id=45842