Roman Szkaradek watched the U.S. Open tennis match between the Polish player Kamil Majchrzak and Karen Khachanov of Russia on Thursday without thinking much of it.

The next day, he woke up to a message from a stranger on social media: “Give back the hat, you thief.”

Mr. Szkaradek, a business owner in Rawicz, Poland, ignored the message. “I thought it was some kind of mistake,” he said.

But as angry phone calls and hateful comments from all over the world overwhelmed his social media accounts and those of his office, Mr. Szkaradek made a horrifying realization: He had been inadvertently dragged into an internet furor over a viral moment at the tennis tournament in Queens.

Videos of a man appearing to lean in front of an excited child to snatch a hat being handed out by Mr. Majchrzak, the tennis player, led to widespread criticism online. Soon, the hat-grabbing man was identified as Piotr Szczerek, the chief executive of Drogbruk, a paving company based in Blaszki, Poland.