A C programming course that I casually enrolled on as an undergrad in 1989. It meant that I had a head-start in learning C++ (cfront) the following year, just as it was becoming the new hotness. Which in turn got me my first two dev jobs and kept me employed for the next decade as a C/C++/Unix dev.

So, you never really know how its going to turn out.

Course materials? I devoured Michael Waite's Quick C Prpgramming book (the one with the pink and yellow cover) in a day, and then the next day I read it all again. Did the same with Stan Lippmann's C++ book the following year, and carried the at&t cfront manual around in my backpack for ages.

I've been working pretty much exclusively in .net for the last twenty years, but C remains my favourite language - even if I almost never write any code in it nowadays.