It was a case of mistaken identity in the lobby of the Algonquin hotel the other day. Sean Hayes, of “Will & Grace” and the big upcoming Broadway revival of “Promises, Promises,” stopped to chat with a man sitting at a table drinking coffee — but then seemed embarrassed and walked away, a witness told Page Six. “He whispered to a companion, ‘Oh wow, I thought that was Stanley Tucci!’ ” It wasn’t.