One time I used Darwin open source to make a printf-sprinkled version of the dynamic linker to track down a bug that was causing our app to not start on about 1

One time I used Darwin open source to make a printf-sprinkled version of the dynamic linker to track down a bug that was causing our app to not start on about 1/3 of our install base. There were no error messages or console logs and the logs produced by feeding dyld environment variables were not helpful, so the ability to build a custom dyld was instrumental. The problem wound up being that we leaked a sticky bit directory into part of our install, the installer inspector hid this from us, and apple changed dyld's behavior in a patch release to silently bail if it saw a directory with sticky bit set.