gistfile1.txt

- (void)setArray:(NSArray *)anArray
{
    _myArray = anArray; // aha! subtle bug because anArray MAY be a mutable array, so you need to do [anArray copy]. With the magic of inheritance, the type system becomes confusingly worthless, telling you that yes, a mutable array is an immutable array. Yay for confusing first time programmers!
}

// This problem exists EVERYWHERE:

- (void)doSomethingWithView:(NSView *)aView
{
    [aView addSubview:controlView]; // This may or may not work depending what aView is. Despite the docs telling you that addSubview is a perfectly fine PUBLIC API, subclasses of NSView may make it effectively private, thus breaking here. So what's the point of saying "give me a view" if subclasses of NSView can willy-nilly change the exposed API?
}