G-20.In is a real domain name, but it hadn’t been used before Mr. Giuliani tweeted it. Twitter automatically converts such valid links into blue, clickable text.

When Jason Velazquez, 37, who owns a web design firm in Atlanta, saw the tweet, he immediately bought the domain for about $5. He then created a page with a simple message, which anyone who clicked on Mr. Giuliani’s accidental link would see: “Donald J. Trump is a traitor to our country.” The whole process took him about 15 minutes, Mr. Velazquez said on Tuesday.

It was the type of throwaway gag that entertains Twitter users for a few hours before forever fading from memory. But Mr. Giuliani preserved it in amber by following up Tuesday with the baseless accusation against Twitter, which has often been the subject of bias complaints from Republicans. He characterized it as evidence that Twitter employees were “cardcarrying anti-Trumpers.”