The specific example I remember (because I'm the one that fixed it) had something to do with how Image I/O handled TGA images. I don't remember all the details, but it was definitely not the case that we were misusing the API. Per the documentation everything had been above board, and everything was still above board. The images causing the issue were well-formed. Just something subtly changed with the CGImageSource API for TGA images specifically that required finding a workaround.

Why that customer was using TGA images in their workflow in the 2010s I'll never know.