Awesome read! Thanks so much for sharing. Love the subtle āUnable to blinkā animation on the POS Terminal 
One of the other issues we see fairly often are offline transactions (which are mentioned as āWhen a merchant doesnāt send an authorisation requestā in the blog post) being declined.
I know that this issue will go away when the debit cards launch but it might be useful if you could confirm that if a userās transaction is declined (when they try to pay at a parking meter, for example) & they donāt see an error message in the app, thatās usually (though not always) the reason - is that correct?
maybe also worth a Monzo member of staff qualifying this query on Facebook post as well
Very interesting.
I had a transaction declined today which failed to show up in the Monzo app at all. It was for 50p, with Ā£10 cashback. The terminal went online and came back with āNot Authorisedā, which is odd - if it was declined Iād expect to see it in the app.
Cashback isnāt allowed for the prepaid cards -
But it will be for the debit cards 
Oh, right. I suppose that must have been the first time Iāve tried it! So thatāll have been rejected at some layer before it got to Monzo?
My favourite decline (What am I saying?? - FML!) was last year paying the Dartford Crossing charge. I called the āDart Chargeā call centre and paid over the phone a few mins before driving over. I gave the card details and both me and the call centre lady ended the call, quite satisfied.
However she must have mis-heard or mis-typed the card expiry date. 2 seconds later got a Monzo notification: āDeclined, the expiry date was wrongā. So I was able to call up straight away and explain that my payment hadnāt worked. So I was able to pay again - properly!
If I hadnāt have been notified of this I would have crossed the Dart in ignorant bliss, enjoyed my 2 week holiday in France and then come back to a penalty charge. OK I might have been able to plead my case as the first call was probably recorded, but it just saved me any hassle and I was over the moon.
Thanks Monzo.
Fascinating read, thanks a bunch for this!
Great post. Thanks for sharing!
I suspected the process was something like this.
When I was in Portugal earlier this year I used my Monzo card on a self-service fuel station.
One thing I noticed as soon as I authorised the card (but even before starting to fuelling up) was that I got a notification on my mobile saying the pump had just charged me for the equivalent of 100ā¬.
When I then finished fuelling up the pump credited the difference.
I understood this a being a security feature for the pump station to avoid being declined an amount after the fuel was dispensed.
But before, on a previous occasion, I was really confused with this next sequence.
There is one shop, where every time I try to use my Monzo it fails with āNot enough TIDSā, yet every other card I have works thereā¦ Any idea what that means and why it happens only with my Monzo?
@anon7743823 TID stands for Terminal ID, they are used to obtain authorisation from your acquiring bank when you try to make a payment on your card.
Each TID can only process one transaction at a time with the same currency and card type. This return code suggests that the TID configured to the payment terminal was busy processing another payment with the same currency and card type when the request was sent.
Itās worth waiting a few seconds and trying again in any case! Iād be interested to know if this works after a second attempt 
Sorry only just saw your reply (and I guess itās irrelevant now, but to satisfy your curiosityā¦), I get the same error every time with my PP card, not matter how many times I retry, and on every time I visit that shop!
My CA card works fine though!
I wonder if prepaid cards need to use different TIDs than normal cards and they happen to have no prepaid-capable TIDs configured, thus causing the error.
I had two transactions declined in a bar last night. Tried Contactless first then using my Pin - same result. The barman just shrugged and was a bit peed off as it was a busy Saturday night. I had to go and find my wife to get her to pay.
There is no information in the failed transactions on the Monzo app. This isnāt the first time itās happened. Each time Iāve had adequate balance. Using the Current Account feels like a roulette wheel 
Have you contacted in-app support when this happens? They should be able to provide greater insight as to the cause, which might avoid this in the future.
If there was no notifications in the app this means the request didnāt even reach Monzo. Most likely a fault in the terminal or their internet connection.