The message queue for DBUS's org.freedesktop.Notifications filled up while I was testing it with notify-send. Evidently, it has a limit of 20 messages in the queue, and if the notification daemon happens to die and not remove itself from DBUS, the messages queue and aren't removed.

After looking into it, the notification interface doesn't have a way to clear the queue unless you happen to know the ID of the message. It doesn't even have a method to list current items in the queue!

I'm hoping that whenever I reboot the machine, it will clear that queue, but until then, I'm using the gnome-osd-client.