Some of your peers are having a little trouble with modifier PC, one of the three modifiers for "never events." The modifier indicates "Wrong Surgery on a Patient," and it will trigger an automatic denial.
Apparently enough providers have used it to indicate the professional component of a service CMS decided to issue Program Transmittal 1867 warning against the improper use of this modifier.
The correct modifier for professional services is still modifier 26. Providers can resubmit claims with the proper modifier.