In an SSL Certificate Error! dialogue, I chose to make an exception for an expired certificate.

I can't find preferences to review, and maybe remove, the exception.

Postscript

I might have found an answer, in that there's no interface; simply no storage of exceptions. KDE bugs:

• 386978 – Ability to store certificate exemptions
• 393162 – Certificate error dialog should be allowed to be ignored on pages

From the latter, comment 1:

If you choose Yes, it will remember the exception for the browsing session (until you close the browser).