I'm on Linux Mint 18.3, with whatever KDE versions are the latest from the official repos as of today the 12th of October 2018.

Occasionally I find there to be what's always a single file transfer that never ends in my system tray. When I expand it with the green arrow on its right there's never anything there. I always have to press the 'Stop' button to make it go away. It seems that my copy/pasting other files somewhere causes it- though it's intermittent, in that I can't make it happen every time I do something.

Screenshot:
https://files.catbox.moe/z6i8r3.png