I use two card processors for credit card transactions, on the grounds that you should never have a single point of failure. I use Barclays and Natwest, via different gateways. And to keep them both alive, I use them both every day.
I put the transactions through Barclays (they're slightly cheaper, I think) and any that fail, I try through Natwest. You'd think that nothing would be accepted, but about 5% are OK. I have no idea why.
So for the last few days, the transactions via Natwest have only been partly processed. And today, I tried a few and they were all not processed. That's not rejected or declined, they weren't processed.
So I called up the people who run the gateway. And they told me that they've changed things, so that after a certain number of declines, cards aren't processed.
I can understand why they did this - it stops naughty people from trying duff cards in bulk. Except that's what I've been doing with them for quite a while now, and I've explained what I'm doing in the past, and they were happy with that.
But the real no-no is that they made this change, not only without asking me beforehand, but also without telling me afterwards.
And that's not playing nice.