Washing dishes, in my opinion, is a system of diminishing returns. As the time and effort put into washing increases, the benefit per resource spent decreases. The first minute washing a plate makes it a lot cleaner and another half hour of scrubbing probably wouldn't do much additional good.
On a related note, I've been snacking from a jar of peanut butter on my desk for the past week or so and have been wondering if eating peanut butter from a jar is a system of diminishing returns. At first, getting a scoop of peanut butter is easy, but as the jar empties, you have to spend more time hunting around for scoop-able areas. Now that the jar is almost empty, it takes almost a minute to get any respectable amount of peanut butter on my spoon. I'm not sure if that meets the technical requirements of diminishing returns or not.
What I do know for sure is that eating peanut butter from jar is a system of diminishing peanut butter.