What to do if you PayNow to the wrong number/person?

Contact that person and ask them to pay you back!
For FIN/NRIC - hard to contact if you don't know them but there are checksum on the final letter of the identifier address so it is unlikely to accidentally occur.

For phone - much more likely to happen coz no checksums/rules on the phone number, but you have the guy's phone number! Call them!

It sure is embarrassing and annoying to claw back but legally the money is still yours...