When designing a user interface, you want to make sure the layout is intuitive enough that people can do whatever they need with ease. When your user is a cat, though, there's such a thing as making things too easy.

The folks at Hiccup found this out the hard way with Games for Cats, the iPad game aimed at our furry feline friends. Like most games these days, the title came with an additional level for purchase. 99 cents for a new mouse level: nothing too outrageous.

The problem was that a lot of cats accidentally purchased the level without permission, which caused a lot of owners to think Hiccup tried to fool cats into buying it on purpose. "We got in a lot of trouble," remarked TJ Fuller, the artist for the title.

Since the developers didn't actually have nefarious plans, they implemented a highly advanced 'human-check' to fix the issue—the need to hold four fingers on touch points without moving them for a few seconds.

I know cats that can do things like open doors and flush toilets: how long before one figures out that the only thing between it and a new toy is standing on top of the iPad while holding still?

5 tips for making video games your cat wants to play [Gamasutra]