It's very unlikely you caused damage with your fingernail. Although it is possible to cause some damage with a strong fingernail, you would have to put some effort into it also. Of course if you were wearing conductive fingernail polish and the computer was on...
No, no nail polish I can assure you
. It was a small tap, basically I was cleaning the area around my rear USB 3.0 slot with my finger and I tapped it. Could a finger nail scratch the inside USB slot or the metal around it?