SecurityAgent hangs when authorizing changes to Certificate Trust Settings

I frequently encounter this totally blocking problem when attempting to authorize changes to Certificate Trust Settings.

This occurs when

1. A dialog is presented when app such as Mail encounters a certificate whose identity is unknown.
2. You choose to show details and select the option to "Always trust (server) when connecting to…"
3. Click Connect
4. A System authorization dialog appears and immediately hangs
5. in Activity Monitor, "SecurityAgent" is shown as not responding.

At this point there is no legitimate way to shut down or restart the Mac other than forcing a power down, since (presumably) SecurityAgent is involved in the normal shutdown process.

After restarting, when the process happens again, I am able to successfully authorize the certificate. It only occurs if the computer has been up and running for a while (hours? days?)