You've handed out a math assignment that was an open unsolved problem and not told your students what you had done prior to them leaving for the day?

Yeah, their "jesting" involved the lecturer, vaseline, a flagpole and lots and lots of string beans.

It strikes me that you expect students to behave that way for challenging them. Maybe it is a matter of cultural differences? In my university, at the most they would complain to the administration. And even then the problem would have to be tied to their grade so they could make the case of unfairness.

My university had a pretty liberal withdrawal policy if you did so prior to mid-terms. Instructors who weren't competent, or who acted like jackasses, didn't get complaints - the students just dropped the class and focussed their energy on instructors who were both competent, and not inclined to sabotage them.

Your imagination immediately jumped to only bad possibilities.