Isn't the English version of that "People in glass houses shouldn't throw stones"?
Me neither to be honest. It's mostly used in Holland, especially politicians seem to find it a a handy saying when under siege from opposition parties/members. Although these days they usually say the first part about the buttered forehead and leave out the "in the sun" bit.
Eh? I don't get that one at all..
Like this variation on "don't throw bricks in a glass house": if you have butter on your forehead you should stay out of the sun.