We only have a couple of weighing machines in each lab and it would take forever for each pair of students to weigh out each of a half dozen reagents at the beginning of each class, so standard practice is to ask the technician to make up some concentrated stock solutions and have everyone add the correct amount of this to a 100ml bottle of agar after the latter has been boiled to sterilise it. The agar is then poured into the plates which are then ready to grow (or not grow - it's a selective regime) the bugs. In baby class we tell the students how much of each stock solution to add, later on we expect everyone to be able to make the calculations themselves.
I have the last section of the week and my colleagues told me they had spent a good part of the class explaining how to do the calculations. I wasn't having any of that. There are at least a couple of conceptually different ways to doing these dilution calculations and I didn't want to steam-roller my way over the whole group, really confusing those who might have nailed another way into their heads. So I gave everyone a little docket and ten minutes to do the calculations in whatever way seemed good. At the end of that I gathered up the attempts anonymously (it's a good thing to know that you've gotten it wrong but it's not-so-good to have everyone else know you got it wrong) and collated the results:
I then let them off to start doing the experiment and while the autoclave was sterilising the agar, they went off for tea. They are trying to persuade the two French exchange students that tea is fit to drink. When they came back - tea really does sharpen the brain, French people - they confessed that they/we would have to dump their batches of agar into the trash. Turned out that Fred (or Delia or Ger) had taken LB broth off the reagent shelf rather than LB agar and that everyone else had weighed their stuff out of the same jar without stopping to read the label properly. They'll never make that mistake again! The clueless are slavish. But if I had hovered over them as they did their work they would never have learned from the mistake . . . and it is entirely likely that I wouldn't have notice that mistake myself.