Re: @Ian Emery
VbV was an early example; and to this day doesnt add any security to your purchase - despite what they say.
No, as I said, I have encountered a number of scripts, some for 3rd party payment systems, some for cosmetic effects, and more than one script that doesnt appear until the transaction has supposedly finished and you are supposed to be sent to the order summery page with an order reference number.
The VERY worst was encountered in the payment system for "The Book People"; it has since been fixed, but was so bad I wouldnt use them for a whole year until it was fixed - and I highlighted the issue on several consumer forums after they failed to reply to my complaints about how insecure it was.