[The Saucepan Man]

From the excerpt quoted by Mister Underhill above:

That dark mind will be filled now with the voice and face of the hobbit and with expectation: it may take some time before he learns his error. (emphasis added)

What is this error that Gandalf considers Sauron may take some time discovering? Surely it is his mistake in assuming that Pippin was the Ringebearer.

It is not at all inconsistent with Sauron assuming that Pippin was the Ringbearer that he would want to question the captive Pippin. Even with the Ring regained, he was still at war with the Free Peoples. He would want as much information concerning his enemy as possible, particularly when the captive was a known associate of Gandalf and had visited Rivendell and (possibly, although this, I think, remains unclear) Lothlorien.

Originally Posted by Mister Underhill

You don't think twice about calling a creature from a species of furry-footed little creatures an "it", especially if you're the most egotistical being walking Middle-earth.

Agreed. Except when you are addressing said furry-footed little creature and directing it to convey a very important message concerning itself to another. In those circumstances, Sauron would not say to the creature: "tell him to send it to me", but rather: "tell him to send you to me".