I just noticed this in the docs for the keyPathsForValuesAffectingValueForKey: method:

Your override should typically invoke super and return a set that includes any members in the set that result from doing that (so as not to interfere with overrides of this method in superclasses).

I've been doing it wrong. I've been doing it like this:

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
    if ([key isEqualToString:@"fullName"])
    {
        return [NSSet setWithObjects:@"firstName",
                                     @"lastName",
                                     nil];
    }
    else
    {
        return [super keyPathsForValuesAffectingValueForKey:key];
    }
}

It didn't occur to me that a superclass might also want to specify key paths that affect the fullName key. A correct implementation would have been:

+ (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
{
    NSSet *keyPaths = [super keyPathsForValuesAffectingValueForKey:key];
    NSArray *moreKeyPaths = nil;
 
    if ([key isEqualToString:@"fullName"])
    {
        moreKeyPaths = [NSArray arrayWithObjects:@"firstName",
                                                 @"lastName",
                                                 nil];
    }
 
    if (moreKeyPaths)
    {
        keyPaths = [keyPaths setByAddingObjectsFromArray:moreKeyPaths];
    }
 
    return keyPaths;
}

Serves me right for not reading the docs carefully. For pride's sake I was hoping this was a common mistake, but from a Google search it seems almost everybody else in the world does it right.

I must have been thinking of observeValueForKeyPath:ofObject:change:context:, where typically you check the given key path against various possible values and fall through to super if it doesn't match any of them.

In any case, I more often use the keyPathsForValuesAffecting<Key> approach anyway.

[UPDATE: Thanks to Christiaan Hofman for pointing out that my description of observeValueForKeyPath:ofObject:change:context: is also Doing It Wrong. So never mind that and pretend I wrote this instead.]