Just like in the UK there are national banks and building societies as well as lots of regional ones in Spain. The "bank" accounts we have are both with smallish regional cajas, a sort of bank cum charitable financial institution.
When we were in Ciudad Rodrigo there weren't any branches of our banks so we used the bank machine of another bunch affiliated to the same network of cash machines. There are three big networks of ATMs and if you use a different network the transaction charges can be very high, up to 6€.
So we pushed our card in a hole in the wall and asked for 300€, I got the receipt but no money. There was an "incident" phone number so I rang them - "No worry" said the man on the phone, "bit of a hiccup in the network the money will go back into your account in a while". Four hours later I rang back because there was still no money. "Odd" he said "you'd better pop into the bank on Monday when it re-opens". I played hell, mumbled about them stealing my money etc., the man agreed. Bit disarming that.
On Monday the bank knew all about it "No worry" they said "it's being credited and should show up tomorrow". The money isn't back yet and I'm going to have to chase it.
But the question is what's to stop the bank simply saying I have the money? The receipt says I have the money. I'm relying on the bank's internal audit processes. I can't prove that I didn't have the cash, I can only rely on them agreeing that they never gave me the cash. I have a bad feeling about this.