However today was one of the few times I'd have preferred a pump out. Two of my three cassettes were out of action - for the same thing, both had the 'holding' tray off its runner by too heavy handed use of the loo brush.
There is only one fix and that involves getting your hands inside and stripping the mechanism to get it all lined up again. I do carry rubber disposable gloves for working on the engine so they were very much used. Rest assured both cassettes had a very good clean out before the open loo surgery took place.
The problem is my third cassette does not have the level indicator on it so it really is the emergency cassette. I did the fix and stowed them away hopefully for another few years trouble (and cash free) use.