Sometimes Plasma doesn't realize an application is running and it blocks access to the desktop
A video says more than a thousand words.
As can be seen, sometimes an application is running but Plasma doesn't realize it is. This only happens after waking up from a locked screen so I'm guessing I've been hitting buttons in my pocket, but it doesn't always happen (actually the majority of time it doesn't).
This application can't be quit, it doesn't appear in the application switcher and the center button to go back to desktop seems to do something in the background but the application doesn't disappear. Because of this there is no way to actually ask the desktop, the widgets on there or the application drawer. Only a reboot fixes this.