IANAL, but this gets complicated. It all depends on whether a reasonable person, when seeing the error page with "no spaces in credit card field", would presume

IANAL, but this gets complicated. It all depends on whether a reasonable person, when seeing the error page with "no spaces in credit card field", would presume that nothing else in the form changed. If so, and the merchant is aware that most people presume that, then it could be fraud. They are making a financial gain, by being aware that the terms have changed, and by misleading the customer into thinking that nothing has changed, they could be committing fraud. The customer choose the original selection, and the merchant's systems swapped in a different, more expensive, version without telling the customer at the last moment, knowing that most people don't re-read everything again, then yes, that's fraud.