Punishable by death in my classroom.
Well, OK, perhaps that's a bit extreme.
In fact, the joke is that I will go crazy if
"Add a zero on the end"is said in my room and I do (but in a good way - lots of shaking of a metre stick, fake hystrionics etc - see example below) and that shared joke is what helps the learners remember that adding a zero on the end is 'bad maths'.
Ask my Year 7 Set 7 from last year. None of them would do it.
They really enjoyed trying to make other people say it though - visitors to the class, teachers from other subject areas, a poor unsuspecting new teaching assistant who joined half way through the year ........
I'm making the assumption that you know what I'm talking about?
If not, it's that common misconception that multiplying by 10 is achieved through "Adding zero" or "Putting a zero on the end". Yep, as an algorithm it falls down fairly quickly with easy examples such as decimals.
The serious side of this though, is just how hard it can be to undo misconceptions and that it's doubly difficult if we, as teachers, have inadverdently allowed our learners to have this lack of understanding.
Hence my major play-acting. Seems to do the trick.
In a lot of cases when learners make a comment it's worth asking
"Is that always, sometimes or never true?"
And don't even get me started on that well-known Rhianna track 'Shine Bright Like a Rhombus'!