@Abhi: Real-world uses: You can essentially use this to write a category that does an override and thus install a workaround to a bug in an OS API (e.g. if you know 10.1 and earlier leak an object in a particular mathod, you can install an override like this on those OS versions to autorelease the leaked object).

It's also handy when you're doing code insertion in another application, e.g. using mach_*, APE or a plugin.