I've never seen this before. I opened gedit (gedit2), three windows but one session, meaning all windows share the same PID. Note that there is absolutely no way to open more than one gedit2 session per Gnome session anyway. At some point during my quick (and maybe somewhat tortured) typing, the window stops responding to any keyboard and mouse input. The pointer (arrow) still changes shape between the text area and the menu etc. area but that's all.
Only for that window. All other gedit windows behave just fine. Running gedit opens a new tab in the "locked" window as if nothing wrong happened, it switches to it, but still no input allowed. I restarted metacity. This often fixes similar problems. No change. So it must be a GTK (GTK2) quirk.
I am simply curious what kind of explanations you could find for this. It doesn't look like a bug to me. For example, is there some GTK keyboard shortcut for locking windows? If so, I guess it can only be an Easter egg since I've never heard of it.