You were halfway to congee.
As you say it should be foolproof, but while you're getting to grips with a new type of rice you can always add too little water and then open the cooker every 5 minutes or so to see what it looks like (or get a rice cooker with a glass top), adding more water if necessary.
You clearly don't have a Japanese (or Korean) rice cooker.
You never, ever open the cooker when making rice with them (they need to build up steam, losing it is disastrous for the cooking process). Japanese rice cookers also have a set of layers beneath the lids - cookers from other parts of the world often have glass tops, but these same cookers are more prone to over-bubbling and scorching rice.