It can be in any browser, but when you get a a certificate warning popping up and you know the site is OK. Is there anything, other than something in Cache, that might be causing this. And is there a possible fix in browser security or privacy that might stop this?

If your browser is set to warn you of invalid/missing certificates, then you will see the error, if it is set to not warn, then you won't see it. It is that simple.

I did reply in your other thread. And turning off the warning is great for when you visit a spoofed site with your credit card and don't see any warning an get ripped grand. We face choices in life.
But pros can forge certificates anyway....

I think your right in the assessment. I believe we have to be vigilant ongoing. That is one way to avoid that, but I believe there is another way. I just haven't discovered it yet. When you know the site is OK, like, then there has to be a glitch in the browser somewhere. Just throwing this out for more thoughts.