Cigar Burn
If you didn't find the carton of cigarettes a very convincing example of abuse and familial dysfunction, you probably will find this one more compelling. It's hard to find a more stunning example of child abuse than a scar from a cigar.
After Andrew says he doesn't really think Bender's home life is as terrible as Bender acts like it is—with Bender's dad yelling "Shut up, B****! Go make me a turkey pot pie!" at his mom and screaming the F-word at him—Bender provides the physical evidence. He shows Andrew a scar on his arm, saying,
BENDER: Do you believe this? Huh? It's about the size of a cigar. Do I stutter? You see, this is what you get in my house when you spill paint in the garage. See, I don't think that I need to sit here with you f***in' dildos anymore!
So, Bender's home life is pretty terrible—that's one thing we can gather from this. And it also seems to indicate that Bender isn't just talk: He's not just pumping up his image. He has the physical evidence of real abuse, and he presents it with conviction. Bender's dad's evidently a pretty sick dude. So, Bender isn't just a goofball with an attitude problem: He's an abuse survivor, and you could argue that all of his misbehavior is really just a way of coping with his situation.
But, interestingly enough, John Hughes intended this to be taken in a totally different way than how Judd Nelson presents it. As Nelson interpreted the character, Bender's dad really did burn him with a cigar. Audiences would agree that that's the way it comes across in the movie, given the way Nelson plays the scene.
But Hughes agreed with Andrew: Bender is all talk. In Hughes's interpretation of the scene (which he, of course, wrote) Bender actually got the scar by catching his arm on a fence. So, it's a good example of how an actor can add a new and different dimension to a scene—possibly to the movie's benefit.