The only time Tubby has ever gone outside the litterbox is when he poops on the rug in the big bathroom - and he only does that when his tummy is upset. I know that's the reason because we will usually find some throw-up nearby too. And this isn't the runs like Max, it looks perfectly normal, it was just that his tummy was upset. With his failing kidneys, this has happened a few times now, but we really don't care because it's on a throw rug and it's poop, which is soooo much easier to clean up than pee. He has always been such a good boy in this department that I just can't see this as a problem. Oh, and we know it's Tubby because I've actually seen him in the act, but it was too late to pick him up and run him all the way downstairs to the litterbox - especially when I know he's only doing it 'cause his tummy doesn't feel good, so I couldn't make him feel worse by interrupting him.

Maybe this or something similar is the reason behind your "incident" because Tubby's problem is much easier to deal with than a behavioral issue. Although, in my opinion, if they're going to go outside the litterbox, what better place than the shower? Easiest place in the world to clean.