Excel is an incomprehensible maze of #defines and macros, PowerPoint is a Golden Temple of overly-object-oriented insanity, and Word is just so old and brittle you'd expect it to turn to dust by committing. There are "don't touch this!"-like messages left near the main loop _since roughly 1990_.
I had a bug in some IME code causing a popup to draw behind a window occasionally, and a Windows guru had to come in to figure it out by remote-debugging the windows draw code with no symbols.
I learned there that people can make enormous and powerful castles from, well, shit.