“There are no stupid questions.”

How often have you heard teachers spout that line to their students, in
an effort to get  them to participate?  I think this is one of the boilerplate
items we’re implicitly taught to use in the obligatory First Day of
Class Lecture.

Well, it’s crap.  Any mathematician knows that a single
counterexample blows an assertion right out the water, so here’s mine.

A student misses one or more lectures (with any one of the standard excuses), and then asks me "I missed class–did I miss anything important?"

Once upon a time, this had the power to upset me.  Now I’m torn
between holding back a snappy comeback ("Only the free pizza and
topless dancer" or "The midterm exam") and stifling a gale of scornful
laughter (snorty-snort-snort-snort, you dumb schmuck).