A mistake or rule broken.
Made famous by Bozo the Clown.
Usually made by a child but sounds funny when you tell an adult they made one.
From an urban legend that is supposedly an episode of the old Bozo kids TV show when a cub scout was playing the spoons game:
Trying to hit one spoon and pop the other spoon lying on top of it into the empty glass for a prize (no one hardly ever won that game cuz it was so hard!). Well...the cub scout lost and said "GOD DAMN I MISSED!" Bozo then stated "That's a Bozo no-no!"...the cub scout stuck out his middle finger at Bozo and said "CRAM IT CLOWNY!!"
Everyone my age has heard of this but no one has ever seen it...thus an urban legend.
Johnny you are coloring outside the lines in your coloring book--thats a Bozo no-no!
Don't eat paste Harry thats a Bozo no-no!!
Didn't you know cussing in church is a Bozo no-no?!
Remember class: sticking your old chewing gum to the bottom of your desk or chair is a Bozo no-no!