It's unclear whether Tay's return was planned, perhaps as part of the planned testing to see whether it was ready to face the public again.For now, at least, the account is dormant again -- unless you're on the followers list already. Those awkward teenage years appear to be far from over for Tay, and like others are finding out, carelessly running your mouth on the internet can be a costly endeavour.

Update: CNBC received confirmation from Microsoft that (as suspected) Tay was inadvertently activated on Twitter for a brief period of time as part of testing.