Glenn, I don't know at this point yet. I was also able to reproduce it on my PC, the first time was on a mac. Both times it happened I was opening a new plan. My toolbars were locked, I added the layerset and dimension default controls to them, and working in both monitors going back and forth with the side windows floating. I think the floating side windows is the key, at least for me. This last time I had just gone into the library browser then went back to the primary monitor did a new plan and bam.