Demonstrating the semantics of __weak objects in Objective-C. See http://clang.llvm.org/docs/AutomaticReferenceCounting.html#semantics for more information.

gistfile1.m

// The weak pointer is read, and retained until the message send completes its synchronous work.
[weakValue doAThing];

gistfile2.m

- (void)doAThing {
    // Within here, 'self' is __strong.

    __unsafe_unretained selfCopy = self;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 'selfCopy' may be garbage here.
        [selfCopy doAnotherThing];
    });
}

Second one is obviously bad, but what about this:

__weak selfCopy = self;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [selfCopy doAnotherThing];

        // selfCopy may point to nil here, never to garbage, right?
        [selfCopy doSomethingElse];
    });

@JaviSoto Yep, your comment is correct.