I’m using the Audio Unit API in Core Audio to play sound on iOS. Since there is a similar API on OS X, it’s been mostly trivial to port the iOS audio code to OS X; for the most part, it just compiled and worked perfectly.
Until yesterday, that is, when I started refactoring the iOS code, adding a Multichannel Mixer unit. It worked on iOS, but on OS X it was silent. After about a day of hair-pulling, I realized that the Multichannel Mixer unit is not actually listed in the documentation as one of the system-supplied audio units on OS X. It’s there in the header files, with all its parameters, and it was instantiated without any runtime errors; it just doesn’t seem to work! I replaced it with a Stereo Mixer, and there was sound again.
[EDIT: It turns out the input and output volumes on the Multichannel Mixer default to 0 on OS X; setting them to 1 lets me use the Multichannel Mixer on both platforms again.]