Apparently there can be multiple nameserver sets for a domain

I recently tried to get the Public Commander Keen Forum back to its classic pckf.com domain after one user noticed that the redirect wasn't working. (Background: the previous owner of the forum didn't own the pckf.com domain, so we couldn't get it transferred during the migration, so a redirect page was placed on the old server instead.) I found that the nameservers had been updated to point to my hosting, so I flipped a couple switches to make the forum available at pckf.com. That appeared to work, but intermittently accessing it would go into redirect loops or throw certificate errors. When there were problems, the domain was resolving to the old server. Some investigation with Google's DNS lookup tool revealed that sometimes the old set of nameservers was returned (so the browser went to the old server), but sometimes the new set was provided (so the browser found the right place). Evidently, there's somehow two differing opinions on what the correct list of nameservers is. I'm not sure how exactly that works, but it's causing problems - I'll contact the domain owner to see if we can get it resolved.