I'm really curious as to how this happened. In the app I work on, all of our debug features are wrapped in Preprocessor Macros so they'll only run in debug mode.
The time the product was released like that, it was a Makefile setting a rather important flag using ?=, in a sub-Makefile two includes deep, and because that happened to have always been the first time CFLAGS was set, it worked. Until I added a new flag. (Solution: don't set your Makefile variables like that.)
When I've nearly done it, it's always been on projects that have the fairly standard Debug/Release build config system (via whatever mechanism), and the Release build is the one that gets released. But then you have no config corresponding to Release-and-some-temporary-debug-stuff, which is something you basically always need. But if you manage this by hand, it's too easy to slip up and have some debug stuff end up in the final product. (Solution: have an extra build config, Final/Product/etc., that is what you use when preparing the final build. So if you get it wrong and leave your debug stuff in the Release build, the result never gets further than other team members.)