So there's an unexplained €10,000 in your bank account - what do you do? Laura Slattery explores the options.

Bank statements are regularly full of nasty surprises. As they push their way through our letterboxes and into our lives, the amount of entries in the debit column documenting our daily travels from ATM to ATM can be shocking.

Surely there's been some mistake, we think, as we examine the crater-sized dent our monthly shopping has left in our credit balance.

But what happens when there really has been some mistake, and this time it is in your favour? Suddenly, €10,000 just appears in your current account, nestling nicely between all those withdrawals.

What do you do?

Do you phone the bank straight away and mutter reluctantly that you're not actually that rich?

Do you start imagining a list of long-lost benevolent aunts who might have managed, somehow, to discover your banking details and credit your account with a one-off gift?

Or do you swing into the nearest garage forecourt on your way home, run your fingers over the polished bonnets and ponder your good fortune?

"Millions of transactions take place in banking. There's bound to be mistakes," says Mr Gerry Murphy who, as the Ombudsman for Credit Institutions, makes rulings on unsettled disputes between customers and banks or building societies.

Your conscience might not extend to safeguarding the profit margins of large banking institutions, but it's probably not a good idea to draw up detailed spending plans just yet.

In most cases, the bank is legally entitled to reclaim the money once it notices that something has gone awry.

"The basic principle is the law doesn't allow you to take advantage of someone else's mistake," says Mr Murphy.

He adds that there have been "quite a few" complaints in this area, with the principle that people cannot be unjustly enriched by a "mistake of fact" often used when settling complaints between customers and credit institutions.

Other mix-ups can occur when banks clear cheques that have been stopped, when mortgage repayments are based on the wrong term and when employers pay their employees by electronic transfer.

"Suppose you get two salaries by mistake," says Mr Murphy. "If the employer realises the mistake on the same day, they can ring up their bank and say, look we've paid everybody twice.

"But any time later, they must ask you to repay the money, because they cannot debit money from your account without your authorisation."

If you stumble across a friendly, malfunctioning ATM that keeps spewing €50 notes without even requiring that you key in the correct PIN, you may be found guilty of larceny by finding or similar offences.

In January, 12 people from Coventry in the UK ended up in court on conspiracy to steal charges for draining a total of £285,000 (€425,000) from such a cashpoint.

There are exceptions to the rule that you cannot profit from banking mistakes, however.

"It all depends on what is reasonable in the circumstances," says Mr Murphy.

If it was reasonable for a person to believe that a sum of money accidentally credited to their account was theirs and they then "altered their position" because of it - in other words, spent the money without realising it wasn't theirs - then the cases is not quite as straightforward.

"If you have 20 or 40 transactions in your account a month, and the bank lodges €200 into your account by mistake, and you don't notice that, then a year later they write to you and tell you they want to reclaim that €200, then they can't do that. It wasn't reasonable that you should notice that extra €200 in the account," Mr Murphy explains.

But if the only activity in your account is the crediting of your salary and a handful of other transactions and €10,000 suddenly appears in your account, it doesn't matter how much later the bank notices its mistake: it wasn't reasonable for you to believe that €10,000 was yours.

If part of the sum has been spent, the bank may agree for commonsense reasons to be paid back by instalments, but it is entitled in law to demand it back straight away.

Perhaps the highest profile mistake of this kind was the case of a Dublin man who received an unexpected windfall from Bank of Ireland in 2001 during a foreign exchange transaction.

Mr David Hickey asked a Bank of Ireland branch in Dublin to transfer 300,000 pesetas (around €19,000) to his account in Spain. The bank transferred €300,000 by mistake.

Upset that Bank of Ireland had called in the Garda, which concluded there was no law broken, Mr Hickey initially said he was going to keep the money and that he had spent some of it. Bank of Ireland then froze the account and threatened to take legal action to recover the money. However, the case never actually got as far as a courtroom, as Mr Hickey handed back the sum in what a spokeswoman for Bank of Ireland describes as "an amicable agreement".

There is no set formula for how banking institutions attempt to recover money credited to customers by mistake, according to Mr Felix O'Regan, spokesman for the Irish Bankers' Federation. "It would be a judgment call within each institution," he says. Institutions would be likely to take guidance from rulings by the Ombudsman for Credit Institutions, he adds.

"The smaller the sum, the greyer the area," Mr O'Regan says. "As far as the actions institutions can take, there are two extremes. Legal action on the one end and 'do nothing' on the other."

Ultimatum-style threatening letters is one option in the middle of these two extremes, but Mr O'Regan believes most institutions will try to look for a mutually acceptable agreement. "I think it's all going to be seen in the context of customer relations," he says.

People suspicious of financial institutions might think that banks sometimes profit from their mistakes, so why shouldn't they profit from the banks' slip-ups? Customers who believe a bank might not be so quick to rectify mistakes in the bank's favour should check their statement carefully every month, according to the Ombudsman for Credit Institutions.

As for mistakes in customers' favour, Mr Murphy believes there's been a change of attitude in recent times. "There's a growing school of people that say 'that's my good luck'," he says.