In message , at
04:59:58 on Sat, 4 Mar 2017, Matthew Dickinson
remarked:
No, you might get an unauthorised overdraft on your credit card,
and quite likely a penalty charge from the card company.

if you got these it wouldn't be rejected, would it?

The two outcomes are mutually exclusive.

Not if the Card Company sends a flag back to TfL saying "that was at
your risk because the customer is overdrawn".

and what on earth does that mean?

A transaction can be accepted in full, accepted at the retailer's risk,
or refused completely.

Any transaction is either authorised, or declined. The only case where
the transaction is at the retailer's risk is if the retailer has
overridden the prompt for an authorisation (a common example is using
Visa Electron on-board trains and planes, where the card must always be
authorised, and historically there has been no way to do so).

That's the situation when the authorisation process simply isn't
available. There's another scenario where (eg) in mail order the
authorisation system says "sorry, wrong delivery address" and the
retailer decides to ship it anyway. Or that's what I've always
understood.

Even if address verification is used, any Customer Not Present transaction
has the potential to be chargebacked.

I'd only expect an authorised transaction to be charged-back if there
was later a dispute about the quality/non-arrival of an item.

--
Roland Perry