While in the bookmarks editor, dragging a bookmark with the mouse, if a
message box (e.g. 'Connection timed out') pops up, the mouse retains the
drag cursor and the whole of X stops responding to mouse clicks. The
message box can still be dismissed with the keyboard and the drag operation
resumes. Presumably this is caused by the mouse being captured by a now
disabled window, so I suppose it could potentially be a GNOME or X bug,
however I'm posting it here because I don't know that, this is where I've
seen it, and it ought at least to have a workaround for now.

I am marking this bug as major because it could potentially cause data
loss: A less experienced user, on finding that his mouse does not work,
might well not think of pressing enter and could resort to terminating X,
thus losing unsaved data in other applications. Besides, hanging, however
superficial, is not what Linux is supposed to do :-).