My very favourite comment was in an AS physics lesson (back when there were such things). The pupils were doing pendulums, and all 3 of them had a different experiment. One each investigated the effect of mass, length and displacement. They worked out that only length mattered, and then they worked out with logic and some maths (and some gentle directing from me) that pi would be in the equation and so would g. I am reasonably sure had they done this a few centuries back they would be in the textbooks. I was criticised muchly for 'not enough progress'. I was further criticised for getting them to do experiments that 'have no point, because you said the mass doesn't matter'.