Random blurry artifacts in cycles rendering.

Hi, I rendered with cycles this short animation and here is two frames that are next to each other. I have no idea what caused these blurry effect to other frame, that is next to the second one with no any artifacts like that

Do you want the file? Here it is:

It seems that this problem occurs only when the pieces are very close of each other and later when the animation is not anymore so subtle, the artifacts don’t occur anymore. Is it a bug with motion blur?

I have the problems with frames 18, 24, 26, 30, 35, 37, 38, 39 and 46 and they render always with these problems even if I render them individually.

Note that the project is done with 2.75a

I’ve tested it and it happens also with 2.77. Noticed also that these problem frames use a lot more time to render than the other frames.

This does not happen in my computer if motion blur is unselected.

Here is my bug report: https://developer.blender.org/T47873

Sergey gave me this note: “The issue with your setup is that the object changes topology, which isn’t supported by Cycles – when using deformation motion blur the topology should match across all frames. This isn’t a bug but rather a known limitation. Thanks for the report anyway.”

Now Do anybody know how should I prevent these topology changes during the animation? I attached edge split modifier to this, but it did prevent these happenings and it was my only idea of how to fix it.

Are my problems really that complicated that NOBODY knows anything about them. :confused:
I don’t wanna bother too much devs with this, because they have better to do than answer user questions. That’s why I try to get response from this forum.