I've seen this happen before. This happens when the post has already been approved /rejected. I have done this many times. I open the link in another tab to check its contents (this was before Pokemaster added the thing where you could see the contents), then approve / reject, and close the tab. When I return to the Admin page, I click the approve / reject button, and get that error.
Most likely you did what I did , or another Editor / Moderator approved /rejected the post already.
And I recognize those posts. I was going through them as well.