context menu does not work on error pages
For instance, per this report from Micah
When you go to a website that doesn't load and instead gives you a certificate browser, and then click the context menu to disable that rule, that website's rule doesn't appear in the list.
Or at least, that's what happened to me when I went to torrentfreak.com yesterday on a freshly installed Firefox in Windows. I think torrentfreak.com doesn't serve the whole certificate chain and I hadn't cached the correct intermediary certificate, so it gave me a warning. When I tried disabling the rule, the TorrentFreak rule wasn't in the context menu, only Google Search was (which also seems strange).
This should be easy enough to recreate on any website with an HTTPS Everywhere rule just by using an intercepting proxy that MITMs HTTPS and serves it's own certificates.