An NSI representative said a "software glitch" made long-registered domains like "microsoft.com" and "news.com" appear to be unreserved. But users could not go on to register the domains, according to the company.
The problem was fixed later this afternoon.
The glitch is the latest in a series of mishaps for the veteran domain registrar. This past weekend, a hacker hijacked educational top-level domains--those ending in ".edu"--including the one belonging to Emory University.
Last week, registrars recalled hundreds of domains registered with NSI over the past few months because they began or ended in hyphens--which is not allowed under domain name protocols.
Last month, NSI and other registrars faced complaints that the universal software used to reserve domains was going on the blink, causing some customers to miss out on domains they were trying to register.