I was playing with shooting beams of super ball, when, much to my surprise, the arc of superballs SPLIT. I was able to replicate the result, and so can you, if you have a load of time. Position the player exacly as you see in the image, and shoot to the left. After a while you will see the split.
I mean the left. They bounce around the screen for a long time before they do that.
Qwerty, you might be right. Maybe it isn't a glitch. Perhaps the line of super balls just compresses enough at that point to start hitting each other. Interesting theory. I thought it probably had something to do with floating point numbers rounding off or something. They had been traveling for a long time.
Edit: Actually, this isn't so rare. I got it to happen in 2 more positions, in just 10 minutes. It seems that, in fact, Qwerty is right. Once they lose velocity a bit, they may compress around that point in the arc and bounce off of each other. A related phenomenon can often be seen with collisions with floors.
I'm sure it was the falling effected by the last bounces into a certain angle that you can find in some places. I don't think this makes much sense, but I'm kind of tired right now. Correct me if I'm wrong.