On Friday, 3 March 2017 14:07:05 UTC, Roland Perry wrote:
In message , at 13:16:27 on Fri, 3 Mar 2017,
tim... 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.
--
Roland Perry

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).