Note: I no longer use key pinning and support for the feature is deprecated in Chrome. Proceed at your own risk.
Public key pinning is scary. The technology’s purpose is to allow website owners to include a cryptographic fingerprint in outgoing HTTP headers that corresponds with the fingerprint of the server’s certificate; if the fingerprint in the headers doesn’t match the actual fingerprint, it could be evidence that something nefarious is going on. You might be the victim of a MITM attack, or the site might have been compromised, or you might be accessing a fake version of the site that has a legitimate (but falsely-issued) SSL/TLS certificate.
The scary part is that as with HSTS, a mistake with your HPKP configuration can make your site unreachable—potentially for a long time.