When your brain is so tired that you go through a particular piece of your XML code, changing all particular instances of < to < and all accompanying instances of > to &rt; ... and then wondering why it doesn't work.
Because "lt" is not short for "left", thus "rt" for "right" makes no sense to the poor compiler.
It's lt for "less than", thus the other one is "gt", or "greater than".
I crashed XMLSpy half a dozen times on that one, actually.
P.S. - I've been dreaming in code, lately. That hasn't happened since college, and it's pretty messed up. I'm sitting there on the couch, trying to keep my eyes open while watching some television show, and suddenly my brain starts fitting the characters' actions into IF-THEN-ELSE structures and other things and visions of angle brackets begin dancing in my head. I find it quite fascinating, almost like lucid dreaming.