An odd one this! We have taken around 50 online event bookings today with no trouble, except for one.
We happened to notice that one contribution for £300 was showing as complete, but in fact the transaction at PayPal (using Website Payments Pro) was for £0.
The contribution has been set to the 'Accounts Receivable' Account, rather than the Payment Processor Account.
A submit credit card payment link has appeared, so Civi seems to know something is up, but to all intents and purposes the transaction appears successful so it was only by chance that we noticed. Is this a bug, or have I missed some intended behaviour?