Monkey patching is how you get into situations where the order in which libraries get loaded causes software to unpredictably work or break. Or, worse yet, to work reliably on the developer's machine and break in the wild. (Because the external library on someone else's server loads slower for the developer than their local library, and faster for some users.)
I think you learn when not to do it, which is most of the time.
Code can end up living a lot longer than people think it will (see Y2K bug).
One of my biggest pet-peeves is how since popular media hyped Y2K up as a potential world ending disaster when nothing happened people thought the whole thing was a myth. When it was not a myth (though it was overblown), it was just avoided because people engineered around it. I can't wait to see how people handle the Year 2038 Problem.
Things I've been reading about python recently makes it seem to me like python people are re-learning lessons from perl.