At my university, our professors take graduate seminars quite seriously. You're expected to behave professionally at all times, it is absolutely unthinkable to attend seminar without being prepared for it, and dressing casually (jeans, hoodies, or sneakers) is not forbidden but it is frowned upon. There is one professor in the department who will allow a *small* amount of off-topic joking or comments in seminar, but in general it is not acceptable.
So, I'm in a seminar, 'Late Medieval and Renaissance Political Theory'. We're discussing the idea of coercive force, and its application in political thought - who uses it, why, and under what rules or authority. We're specifically discussing warfare. A student, M, pipes up to add his contribution to the discussion. EB is a girl sitting next to him, who is a close friend of mine. M is a mutual friend of both of us. EB and I are both prone to bouts of the Inappropriate Giggles.
M: Well, after a battle, the distribution of booty was -"
EB: "HA" *snort*
Class: Horrified silence.
EB, panicking: "Oh, I'm so sorry... it's just, he said booty... it was really funny, I'm so sorry, that was inappropriate, please go on."
Me: *struggling to contain my laughter at poor EB*
M, deliberately: "As I was saying, the booty was crucial to authority figures -"
EB: *snortgigglechokecough* Makes the fatal mistake of making eye contact with me.
Me: *collapses into fit of Inappropriate Giggles*
EB: *collapses into fit of Inappropriate Giggles*
Professor: *not amused*