C or D: that 5 minute section of an otherwise dry series of lectures where the tutor(s) uses humo(u)r to keep everyone's attention

example:
http://farm6.static.flickr.com/5289/5211607101_15a0ef9e4a.jpg

I'm going to say DUD because it breaks my concentration, which is easily enough broken anyway.

I had an Intro to Psychology teacher who claimed himself to be a comedy writer in his free time. He spent most of his lecture time making unfunny psych-related puns that received no reaction from the students.

Actually I have one example where it isn't dud. A secondary school history teacher who interspersed his classes with stories about all the different jobs he used to do, including hawking dodgy gold bracelets Del Boy style in a market. But maybe I think of it as not dud because I opted not to take history for the final two years of school. He also had some good advice about that, which he told the entire year in an assembly "If you want to avoid a lot of work, don't take History. Do Geography instead."

And also from secondary school, a resoundingly dud example - a teacher who tried to be 'down with the kids' by talking about TV shows that he liked to watch.

(which is a slight deviation from the thread title, because he used to do that all. the. damn. time.)