I saw some very cool looking cirrus clouds high up, but there was a strange kind of shape in them
Problems with rendering transparent layers which are not parallel (which the Cirrus clouds aren't, because they're curved sheets) under some angles leads to such problems. I've tried to circumvent it by trying to make sure that Cirrus clouds don't overlap much, but occasionally it still happens. The problem is either in OSG or even deeper in OpenGL, but there doesn't seem to be anything we can do about it. Has on occasion also been seen with scenery objects, just less spectacular.