When not all of the lecture gets recorded.

“Dude, do you remember what opsonization was?  From lecture yesterday?”
“… Oh, uh… it had to do with complementation, right?””
“What was that?
“… antibodies?”
“Right, ok, so I think that antibodies got.. coated.  And then.. I don’t rememb-”
“OH!  Right, so antibodies get coated and then some shit goes down and somebody gets phagocytosed.”
“… Right.  That.  Exactly.”
“… We should study together more often, dude.”