One of our testers noticed a graphical glitch with the frozen column splitter and I've been able to see the same behavior in your demos.
If you go to http://demos.telerik.com/silverlight/#GridView/FrozenColumns and start to move the frozen column bar, if you right click before releasing the left mouse button, the gray splitter bar remains permanently visible.
If you go to http://demos.telerik.com/silverlight/#GridView/FrozenColumns and start to move the frozen column bar, if you right click before releasing the left mouse button, the gray splitter bar remains permanently visible.