The game area turned completely black when I merged my two lvl2 skulls (with bones) today. I suspect it had to do with the achievement that it tried to give me.
It was the first time I played on this version and the first time in that game that I got a unit to level three. I was playing on Newgrounds and It showed a notification about an achievement that I got. However, I was not logged in to Newgrounds, so I couldn't actually get achievements. I was playing on Chrome.