If there are two buttons at the same layer, newbies or even pros may have problems to figure out on what button can be clicked. Because the engine shows the latest created button higher than the button created before. Could the compiler give a warning, when two buttons are covering themselve.

The compiler can not know when buttons overlap; the engine could, but as you're then seeing that anyway such a feature is not needed, especially since it would be CPU intensive.

I see, you have to work more exact with these things. ^^