When is a transaction not a transaction, but is a transaction?

 I have to pay the government money.  This particular bill allows me to drive my car on their poorly maintained roads.  So I use the 16 digit id and up come the charges.  I fill in my payment details, and the bank (the one with a blue logo) wants to be sure I am who I am.  The pop-up is blocked by my ever vigilant browser (FF), and a long and detailed error message appears on the web page.  Something about the transaction cannot be completed due to an error.  It went too quickly to save, although that may have been my fault, I'm not sure.

Still the transaction is not a transaction any longer.

I try again.  It says try in 5 days.  My confirmation email comes through, saying my vehicle is taxed.  I check the gov web site using my registration number - no, it's not.  It still expires at the end of the month.

 I log on to my bank, and there is the pending transaction.   Will it go through, or not?

Five days later. I try to give them money again, but they say it's already gone through. Checking my bank, I see the charge is on my account.

 The transaction that is not a transaction is now a transaction.

Conclusion - Bank security is rubbish. DVLA error processing is suspect.