One of the things we learned about in class was route summarization (area aggregation in "OSPF speak") -- a handy thing if you have a lot of routes in your network, and want to trim the work the routers have to do in keeping it all straight. Route summarization is kind of like telling the routers that instead of telling all of the other routers out there about every telephone exchange in the area-code, they should just advertise what area-codes they service.
With route summarization rules in the routers that join different areas of the network together, I cut our routing tables in half. With a couple of overly ambitious summaries in the core, I created a routing loop that broke part of the phone system. Thankfully the fix was as simple as deleting the errant rules. As Pat says, fixing problems is how you learn! And creating problems is how you learn even more!