I had a class in grad school called Database Implementation. We wrote a simple SQL database from the ground up, with a SQL parser as the first assignment. I had worked in industry for a few years, so it was fun seeing that "I have painted myself into a corner" look in some people's eyes. And the "I have to rewrite everything" scramble. :-)

That was also when I learned you could deadlock AIX 3.2.5's NFS filesystem using mmap. (I was also working as one of the dept's sysadmins, so putting 2 and 2 together was easy.)