This emerged during the pre-testing of Plasma 5.10 beta in openSUSE. notify-osd and plasma-workspace install the same DBus service file (in this case /usr/share/dbus-1/services/org.freedesktop.Notifications.service) and thus clash. This may prevent inclusion in all distributions which consider conflicting files as a fatal error (openSUSE is one).
Pushed this morning, didn't see I had a bug report.