How does score verification for subject tests work?

Is it for all 3 tests or just the one that I specify?
Also, is it common for the machine to have any irregularities? I specifically remember erasing a lot on that exam and I might not have erased every single choice completely.
I’m just wondering what the odds generally are for this and if anyone has tried it before and gotten a better score back.
I compared a lot of my answers on CC and got about 15 wrong, but based on my score it seems that I have gotten possibly 30 wrong. Does the score verification still work if you didn’t perhaps erase your answers 100% and bubbled in another answer?

Thanks