jsContract

The following script is loaded on this page


	var instrument = (location.search && location.search.indexOf("instrument=true") !== -1);
	Contract.load("MyClass.js", instrument, function(){
	    try {
	        var myClass = new MyClass({
	            mode: "multiply"
	        });
	        
	        var result = myClass.publicMethod(34, 5, 3);
	    } 
	    catch (ex) {
	        alert(ex.message);
	    }
	});

When run without instrumentation this will not create any exceptions, but when run with instrumentation an erroneous postcondition will come into play.
The offending code is

	Contract.guarantees(function(result){
	    return result === 0;
	}, "Result must be > 0");