It's true. Take for example this morning when I upgraded sendmail innocently on my server only to discover a bunch of things suddenly stopped working. Some of the things were things I didn't even know people were using.
Hours of tailing the maillog and poking around the sendmail site, I discovered all kinds of exciting stuff about configuring sendmail for smtp authentication that I didn't know about.
Every time I upgrade Apache, I learn all kinds of new things as well. The only thing I wished, is that it wasn't quite so trial-by-fire. Of course, for that to be the case, I would have to know everything and there's a whole lot of stuff out there that I'd rather learn posteriori rather than a priori. Otherwise, I'd just spend all my time studying things I would never have time to use.