Good question
This is the first model that used a HEPA exhaust filter, and while the filter fit the compartment, there was room between it and the vacuum body. Looks like the filter got so clogged that dirt had no where to go. I'm willing to bet that they either ran it with not enough water in the pan, or no water in the pan. The old filter was stained so it appears that they may have over filled it at one point. I've NEVER seen one of these so dirty. I also believe they left it full of dirty water because the mud in the water pan was dried up like cement, and difficult to get cleaned out, but I did it. Obviously the people who had this machine before me either had no idea how to take care of it, or as I suspect, didn't really care. I can guarantee that this will never happen to this machine again because I do know how to take care of it and will perform routine PM to keep it running.