I have run into the same issue with all versions of 2012 Pro, not just the service releases.

It happened in 2010 Pro as well.

I turned off multithread bending and it seems to have corrected some of it, but not all of it.

Another thing I noticed is that if you leave the setup room after working with each bone (grouping, joint, etc) then go right back to it, it happens far less often. Something tells me that the issue starts when you modify more than one bone/group, then leave the setup room, it will corrupt one of them.

I have a few error logs that I need to send to SM that show a NT kernel access error when it happens as well. I am not sitting at that computer right (at work right presently) now to list the entry point that causes a few issues to crop up as well. Once you get the kernel error to happen, you have to reboot the machine.

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