A few days after my R exam (available in French here), I received this email from the department secretary:

Pour information cet étudiant est venu me trouver jeudi soir, catastrophé car il venait de constater que son stylo “PILOT Frixion”, stylo semble-t-il recommandé par certains enseignants, ne se contentait pas d’effacer la page sur son recto mais également sur son verso, rendant ses copies illlisibles… [For your information this student came to me last Thursday night, looking devastated, as he had just discovered that his “PILOT Frixion” pen, pen that seemed to have been recommended by some teachers, did not only delete the page on its front but also on its back, making his copies unreadble]

That seems like a bad joke, I cannot believe that (a) any math teacher would recommend a special pen brand, (b) the student was using this pen for the first time, (c) he did not check his exam copy before handling it back. Maybe we could design an experiment to test how quickly the eraser goes from front to back!