Visual Studio mystery line

From time to time a strange thing happens to my Visual Studio - a strange line appears on the screen in a random place and stays there until VS is restarted. It is only visible in VS, when switching to other programs it disappears. Switching between windows inside visual studio does not help.

This is just one of the hidden and secret features of the VS that most people don't know about. The other one, a more common one, is the fast disappearing feature which makes VS dissapear (self terminate devenv.exe process) at random when you are working.