In the letter that Gandalf asks Barliman Butterbur to send to Frodo, he says

Do NOT use It again, not for any reason whatever!

This letter was written on Midyear's Day, almost four months before Frodo set out for Rivendell. The first time that Frodo wore the ring, as far as is evident from The Lord of the Rings, was at Tom Bombadil's house on September 27. Why, then, would Gandalf tell Frodo not to use the Ring again if he never had before? Did Frodo use it at some point earlier that just isn't stated in the book?