It's not just the fence joke. George Lopez got physical with the guy because he felt that insulted by the depths of his thievery.

And yes, people have similar premises all the time. But if multiple people are calling you a thief and instead of denying it, you spend time calling them jealous...you're probably not just coming up with the exact same joke as all these other people.

And even if he didn't steal that Cosby joke, he's still kind of a slacker for not being familiar enough with "Himself" to realize that it's already been done.