I was so tired yesterday (i haven't been sleeping well), that I walked into a class room and stood there for several minutes looking at the instructor, who wasn't my instructor, although he was talking about something familiar, as if he was subbing for my professor.  And I looked around the room and the faces were familiar (they were other math grad students), but I was like, something isn't right here.  It doesn't seem like enough students.  And that isn't the right prof.  So I asked - Is this 511?  (The class I was looking for was 505, but I was tired, I couldn't even remember the right number!)  The prof was like nope - 547!  I said SORRY!  I am so lost and bolted from the room.  I found my classroom a few doors down.  I have no idea how I got so lost, except sheer tiredness.  But in retrospect, it was pretty funny!