After seeing the correct version the problem is really obvious.
The particular problem here is that at some point in the pipeline the colour of the particles were stored linearly instead of in sRGB. That results in the colour banding.
A programmer who was working on the engine managed to accidentally break this, and the thing that really sucks about it is that the issue is really subtle and nobody noticed it for quite some time. Thankfully it didn't get to production before I noticed it, but that was complete chance. It could easily have gone out like that.
Look at the particles inside the pit at the bottom of the screen. http://imgur.com/BQjJD25
Can you see what is wrong?
Now take a look at what it used to look like: http://imgur.com/9z9b1cq
After seeing the correct version the problem is really obvious.
The particular problem here is that at some point in the pipeline the colour of the particles were stored linearly instead of in sRGB. That results in the colour banding.
A programmer who was working on the engine managed to accidentally break this, and the thing that really sucks about it is that the issue is really subtle and nobody noticed it for quite some time. Thankfully it didn't get to production before I noticed it, but that was complete chance. It could easily have gone out like that.