Back in the stone age, when I was in high school, we had a guy in our D&D group who did this all the time. He’d roll a d20, get some mysterious number that no one ever saw, then smack said die away in disappointment while telling us he missed.
Then, you could see the gears turning in his head as he added up all the “modifiers” in his head. He’d pause for a moment, and then say something like, “Wait, I dunno, maybe this DID hit…17?” It usually did.
I’m not a big fan of munchkins when role playing (and lord, are they everywhere!). But I’m even less of a fan of cheaters. It’s a GAME, for crying out loud!