In some situations the plasma session will not shut down.

for example: install telegram messenger -> https://telegram.org
login and let them open.
now when you try to log out, restart or shut down your computer it the result is that the desktop will go black, kwin will stay alive and your computer wont shut down. you have to switch to a TTY and shut down from there.
there's an upstream bugreport: https://github.com/telegramdesktop/tdesktop/issues/633 but the real problem is that we sit on a black screen, when try to shutdown. it it would be canceled and we still are on a usable desktop, it would not be that bad

next thing is tomahawk media player -> https://www.tomahawk-player.org
in some situations (not always, like it is with telegram) when you try to log out nothing happens. you can click on the logout button in your menu, the logout screen appears and you can click on log out and thats it. no app will close itseltf

the last thing is kate (our kde kate)
when you open some document, write something and then i try to shutdown my PC, plasma ends itself, kwin closes itself but kate asks me if i wanna save my work. so plasma and kwin really should wait until i save or close kate, otherwise i'm on a black desktop who dont shutdown itself anymore

tested with archlinux (always latest framework/apps/plasma), Kubuntu 15.04, 15.10 Beta, Manjaro, Fedora