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var Test = new Class({
    'Aspect':'Class returns wrong data',
    returnFour:function(){
        return 3; // HEY! This is an error!
    }
});

    AOP.addEvent('post', function(e){
        if(e.name=='returnFour'){
           e.returnthis = 4;
        }
    });

var a = new Test();
alert(a.returnFour());//4

<script src="https://raw.github.com/keeto/mootools-pattern-mutators/master/Source/Class.PatternMutators.js"></script>
<script src="https://raw.github.com/gist/1278659/64ee9462dec56a74e0db9ee3f01db3c62f9dd957/kenta.AOP.js"></script>