Dumb, simply put, is when you don't know any better. This could be on an intellectual, mental, or educational level. It could also be due to a lack of exposure to certain behaviors and subsequent reactions, relative to any kind of 'life' situation. The rate at which you react and whether or not you think first before the action or reaction is somewhat relative. Definitively though, nothing is more evident after the fact, or nearly as important as the simple fact that you didn't know any better at the time.
Stupid, simply put, is when you know better, but you do it anyway.
The first time you do your own laundry and toss bleach into a load of colored clothing, destroying everything... this is a dumb mistake. The next time you do laundry, and toss the bleach into the load of colors again? That is just stupid. You knew better from the first/past experience, but you did it anyways.
Usually, we all have our first good drunk when we come of age. How horrific is that feeling of being sick for days? It is a DUMB mistake, or so we tell ourselves until we have survived it, recovered and forgotten? Somehow we convince ourselves that as we come of age, we develop cast iron stomachs and grow ten foot tall and bulletproof. So, as life should have it, we go out over and over through our life span of countless happy hours and parties... getting STUPID drunk. That's right, we not only know better, we know we are going to feel like total hell... but we do it anyways.
A few years back, an extremely intelligent friend of mine was mixing cake batter. The mixer was plugged into an extension cord, which was, in turn, plugged into the receptacle. After she was done she unplugged the mixer and as she did so, the end of the extension cord that it was plugged into from the receptacle, dropped into the batter. To my surprise, she picked it up and decided to lick it off! To her surprise it decided to lick her right back. She obviously forgot that the end of the extension cord was still plugged into the wall! I can tell you that it was a shocking experience she will never forget. You can tell me... dumb or stupid?