cfv tries to parse any .p## file as a par file. You would get the same error if you did s/a.p12/a.sfv/, though I agree it's a bit more annoying with this case since it's not immediately obvious what a .p12 or other .p## file is. Maybe there is a nicer way to handle this, though I'm not sure what it would be.
2009-11-06 07:08:21 UTC in Command-line File Verify