I've noticed lately that some window titles in the taskbar are replaced with the current time as displayed in the clock plugin. I've not been able to figure out what triggers this but if the title of the affected window changes the clockiness is cancelled and the title returns to normal.
I've had similar bugs and they all went away. My best guess is they were triggered by the new client-side-window code in gtk. Please reopen if you still have this problem with the latest gtk and panel.