This isn't for everybody, but I hugely enjoyed it, and I think some of you will, too:
The never-ending conundrums of classical physics
Even today, scientists still try to solve sprinkler brain teasers from the 19th century.
I remember having a discussion with several other bright undergrads, our TA, and one of the physics profs about the sprinkler problem (we were all reading Feynman at the time), and we came to ... no conclusion. (I managed to end the discussion by speculating that the sprinkler would move chaotically, which, while incorrect, at least had the advantage of being a proposal no one else had made.)
I also remember the feeling I had at about the same time -- a few weeks into my Intro to Modern Physics course -- that the message of the course was everything you have previously learned about physics is wrong.
As with most things, it turned out to be not quite that simple.