I only fiddle around with Opera a bit to check compatibility on different sites I’ve developed, so I’ll admit I’m not an Opera guru, but I did manage to fix a bizarre issue that caused some frequented sites to show up with an error saying it was blocked, even though I was sure I had not blocked it.

To undo the block, right-click on the page, select "Block Content", and click the "Details" button to edit the list of blocked content.

So of course the first thing I do is try what it suggested, but the sites were not listed as being blocked. So I tried adding them, restarting Opera, then removing them. Nope, that didn’t do it either. I found a few threads discussing the same issue and they suggested editing or wiping out a certain config file to reset it. I didn’t really feel like doing that so I clicked around some more and stumbled upon the solution.

1. Right Click (on affected site)
2. Edit Site Preferences…
3. Content Tab
4. Uncheck “Enable content blocking”
5. Click OK

So it was a fairly obvious solution, but not at all what the error screen was telling me what to do. Good one Opera.