Always count on me to find unique ways to break things. While importing the U8glib library, I accidentally selected my entire download directory instead of the .zip file (this is under Win10, BTW). It choked, and that's an understatement. I couldn't even get taskmanager to acknowledge that the IDE was running, let alone kill the program. After a cold boot, the IDE came back up - I thought I'd hosed it - and there in the Library list was a gray Download listing. Can't get rid of it. It doesn't appear in the Library Manager. I seem to recall that removing something from the list has been a problem in the past.
This ability to select an invalid target has me bothered, though. The IDE died the second the directory was submitted. I can actually see a purpose behind the ability to enable directory selection, but there probably should be some additional error checking...er...to prevent ingenious fools like me :)