It will go wrong, it always does.

The EU (and the UK) are proposing a tax on banks to cover banking failure, ostensibly it's to prevent the taxpayer from having to bail out banks and there are proviso's to make sure the cost of this tax isn't passed on to the borrower (some hope, they'll get their money back somehow, it's what banks do)

BBC.

A network of national funds should be introduced so the cost of bank failures are not met by the taxpayer, the EU internal market commissioner has said.

Michel Barnier said such funds would provide part of a broader system aimed at preventing future financial crises.
Banks would be required to pay a levy into the funds which would not be used to bail out failing banks, but manage failures in "an orderly way".
Mr Barnier said: "I believe in the 'polluter pays' principle."
"It is not acceptable that taxpayers should continue to bear the heavy cost of rescuing the banking sector. They should not be in the front line," he said.
And the EU report said that any levies that banks were made to pay should not be passed on to their customers in the form of higher charges.
UK Chancellor George Osborne has favoured a banking levy but would prefer national governments to have more freedom to decide how the money is spent.
Commenting after a meeting with US Treasury Secretary Timothy Geithner on Wednesday, Mr Osborne said he agreed on the need for a bank levy to fund the cost of future failing banks.
But he reiterated the UK's position that the funds raised should be treated as general government receipts, rather than as a specific ring-fenced fund to be held in place until a crisis arose.

Sounds great doesn't it? However like any tax, sooner or later it gets used as a "temporary" top up to other schemes. Bit like road tax by far exceeds what is spent on roads, it's just another tax all ending up in one big tax pool. That's the issue that I have over this, the fund will be administered by the government and frankly I don't trust the government, their intentions might be good, but sooner or later some unscrupulous chancellor is going to come along and see all this nice "just in case" money sitting there and think well I have a few schemes that could benefit from that. Next time the banks have a problem, the money will be gone and the taxpayer will be hit again.
Again, I might be wrong about this, but the past track record of governments isn't a good one where it comes to keeping taxation for what it's supposed to be for. National insurance, road fund license etc all collected yet not used on what they are supposed to be for, simply government funds. Now I'm all for the banks having to pay for one of their own going bust, but I'm more in favour of an organisation along the lines of ABTA, to look after the problem, not the Bank of England and hence not the government of the UK. Politicians simply cannot be trusted with money, they always find a way to spend it, not save it.