The other day I tried to load a URL in Safari that has worked dozens of times before, but now Safari prevented it from opening entirely, instead presenting a dialog

The website “[my requested URL]” requires a client certificate.

This website requires a certificate to validate your identity. Select the certificate to use when you connect to this website and then click Continue. In the window, it presented only one certificate to choose from so I selected, it and got the exact same error again, immediately. The certificate that kept showing up in the window was which apparently is the source of many problems.

To fix the problem,

  1. Go to Finder > Applications > Utilities >Keychain Access
  2. Search for the name of the certificate(s) that showed up in the certificate window above, and delete them all.
  3. Then restart Safari and try again.