Has anyone found a solution to this?  I'm getting the same problem.  Just type something in the editor and then ctrl+z to undo it and I get the error.  It even does it on the demos on this site, even the editor that I'm typing this post in will do it.  It seems to only be in IE though, or at least it doesn't do it in Firefox.  Also, ctrl+y to redo exhibits the same error.

Thanks,

Zach