''They didn't deal with the problem .. so they made regulations. '' _Broken

Why some users can not just figure out the design? Are they stupid or what?
Some clues regarding this I found in an scientific article. But the following is just an example of everyday life.

It was spring time and I was in a space looking like a greenhouse with high walls, which was also hosting an exhibition. It was located in the middle of a nice green area, a huge park. Later on I was informed that this was a museum.

The scenery was amazing. The light could go through the glass and the green colour of the trees could just be seen reflected everywhere in the surfaces around.
The setting was very minimalistic. There were colourful round plates of different sizes on the floor or hanging from the ceiling. That was the installation.
Kids were running over them, one could see the footprints on almost all of these pieces of art. Girls were posing for pictures standing on them or couples were chilling sitting there. Most of the people would just come in take some pictures and go or observe the view out of the big glass construction towards the park. I assumed that it was such a successful setting for chilling out there, and playing open to everyone.
It was beautiful.

I just love glass, reflections, the play of light and shadow, and the green so I just thought I would sit back in a corner and observe these.

You are not allowed to sit here
 
But no! A few minutes later a security guard came. He was hired to tell the people that they're not allowed to sit anywhere in this space. No chair was in there even for the old. I then noticed. At least they tried. So this place was not for sitting, or running, nor playing. The plan was to walk through, enjoy the art, maybe take some pictures and walk away again.

A space designed in a way that invokes a specific behaviour (of relaxation) but maybe not the wanted from the ones in charge, so they hire someone to control that behaviour. Someone to tell what was not enough perceived by the intuitiveness of the design.

In this video Seth Godin. This is Broken. gives a number of interesting examples where the designers or engineers didn't take in consideration some aspects.
This is a site about product design - reports on 'broken' products.