I was also expecting to see an explanation of why the crash was occurring, given the build-up. Here's my theory: One or both of the parameters were passed in

I was also expecting to see an explanation of why the crash was occurring, given the build-up. Here's my theory: One or both of the parameters were passed in from unsafe references, and the objects had been deallocated. According to a simple reading of the code, you would not expect this to be a problem, because the method doesn't attempt to access either of them (it's equivalent to passing an invalid pointer to a C function, and then not accessing it). However ARC subtly changes the semantics of the language. Strictly speaking, the parameters are supposed to be valid, as ARC adds retain/release calls for them (the fact that those are normally optimised away is purely that - an optimisation). So the bug here was that invalid pointers were being passed to a method which does actually use them (albeit non-obviously). As soon as retain was called on the invalid pointer, it ended up accessing or calling to some arbitrary location in memory, hence the crash.