Some registrars have done something called "domain tasting", where they will register a domain when they see interest via their web whois tool. Then they will offer it for a higher price. They were able to register it for five days without paying anything, and then cancel the registration if they didn't manage to sell it.

This has mostly stopped since about 2010 when registrars had to start paying a few cents per transaction. Since the overwhelming majority of those never converted to a sale, it's no longer really economically viable and I don't think it's much of a worry today.