Occlusion Culling: Preview vs. Production

When I choose "Preview" mode for baking my Occlusion Culling data (rather than "Production" mode), I understand that this is less accurate somehow...

But what does that mean? Will there ever be "false negatives" from Preview mode - i.e. things that should be visible and are not? Or will there only be "false positives" - things that should not be rendered but which are?

Hopefully the answer is the latter - that there's extraneous rendering going on. I was getting some issues with things being invisible when they shouldn't be due to some occlusion culling problem I had, and it got me wondering whether that wasn't caused by using Preview mode.

It really depends on your geometry. I have had some scenes that work fine with preview mode and others that need production mode. Now, I will I just set it to production, practically all high settings and let it calculate overnight for anything that I'm doing.