How do you remove SSL certificate exceptions on OS X? When connecting to a URL via HTTPS and the SSL certificate doesn't match (such as at a paid Wi-Fi hotspot), Safari shows a dialog asking whether the certificate should be accepted. If you accept the certificate, OS X adds an SSL exception and will never ask about that certificate again.

The question is, where is that exception stored? And how can I get rid of it?

Note that this question is essentially the OS X version of “How do you remove SSL certificate exceptions on iOS?”, however the iOS answers make no sense for OS X.
The exceptions are stored in Keychain Access. The certificate will be copied there, together with the trust settings. Double-click a certificate and you will be able to change it's settings. Note: you can have trust setting on system level and user level! They have separate keychains as listed in the sidebar.