Swift code can crash at runtime with EXC_BAD_ACCESS when you use a nonnull property as an @optional requirement in ObjC that mistakenly returns nil.
This happens in debug and release builds with unguarded access to that property, and only in release if used in an if let x = obj.objcProperty.
It only seems to occur when the property isn't a bridged type (it doesn't crash with String).
The crash output looks like this:
See the attached project for a more nuanced example (ViewController.swift). Note that you will have to run on the debug or release schemes to see all effects.