In the summer of 2012, thousands of people woke up one morning and found that they couldn't access their bank accounts.
And this wasn't just a small technical glitch to be fixed quickly.
Some 600,000 Ulster Bank customers found that they couldn't pay their bills, get into their accounts or receive their wages.
Many enjoying a trip abroad were suddenly stranded with no money, and 30,000 social welfare recipients were also hit. Labour Senator Lorraine Higgins succinctly summed up many people's feelings towards the bank, branding the failure "a fiasco".
The problem was caused by a software update applied on June 19, 2012, to the systems of Ulster Bank's parent company, the Royal Bank of Scotland.
What should have been a routine IT update instead ended up being debated in the Dail after the bank's system was corrupted.
Many customers were still having problems accessing cash two weeks later, forcing Ulster Bank to admit that it was unsure when normal service would resume. Despite attempts by the firm to quell the chaos, many were still experiencing problems nearly a month later.
After appearing before the Oireachtas Finance Committee to discuss the situation, the Central Bank launched an investigation into the errors that plagued the bank, which it concluded earlier this week, fining it a record €3.5m.
We reveal the largest fines that the Central Bank has handed down since the financial crisis.
Quinn Insurance €5m
Although Quinn Insurance's fine was bigger than the one against Ulster Bank, the reason why Ulster is being heralded as the not-so- proud-owner of the Central Bank's largest-ever penalty is because Quinn Insurance was unable to pay its fine.
After a first massive Central Bank penalty that led to Sean Quinn stepping down as chairman of the company (more on that later), by 2010 the Financial Regulator had had Quinn Insurance put into administration.
Because of this, the Central Bank waived the fine - the largest amount it was capable of giving until new regulations doubled that amount. At the time, a spokeswoman said: "It would've just ended up with us taking money from the taxpayer."
Ulster Bank €3.5m
There isn't much that hasn't been said about Ulster Bank's disastrous IT failure. The debacle eroded confidence in the institution, resulted in thousands of customer complaints and cost the bank at least €100m.
Ulster Bank chief executive Jim Brown has admitted that the bank let down its customers, saying that it "went to the heart of the trust they have in us as a bank" and pledged "we are quite clear that they should never have to experience anything like this ever again."
Quinn Insurance & Sean Quinn
In 2008, Quinn Insurance received another huge fine of €3.25m, and Sean Quinn Senior was hit with a personal penalty of €200,000, leading him to step down as chairman of the company.
The former billionaire had taken a loan of more than €280m from Quinn Insurance to cover losses incurred by, and buy more shares in, Anglo Irish Bank.
Quinn had built up a substantial stake in the bank, as much as 28pc by 2007, but lost billions when the bank was nationalised.
At the time of the ruling, he insisted that his building up of the stake did not make him a "gambler" - despite the fact that roughly €3bn was wiped off the family's wealth.
He did admit, however, that he had acted poorly: "I shouldn't have taken money out, I was a bold boy and I've always accepted that".
Combined Insurance Company of Ireland
The Irish branch of international niche insurance firm Combined Insurance Company of Europe was hit in December 2011 with a then- record fine of €3.35m after 28 breaches of the Consumer Protection Code. Customers were being over-insured, and, perhaps even more seriously, the firm was also found to have used clients' bank account details to open up policies in other people's names.
The group was ordered to refund €2m to policy holders, and the debacle led to the entire board of directors and management team being replaced.
Alico Life International
Alico Life, a Dublin-based subsidiary of insurance company AIG, was hit in the pocket for a whopping €3.2m in April 2012 after getting caught up in the securities lending fiasco which threatened to engulf its parents company.
There were several regulatory breaches, but perhaps the most serious issue cited was Alico putting €138m that was beneficially held for clients into mortgage-backed securities, which ultimately triggered losses of €42m.
The firm would have been in breach of its solvency requirements were it not bailed out by the Federal Reserve after parent company AIG lost €15bn on securities lending.
The company was also unaware that a portion of its life insurance fund assets had been loaned out by an agent on behalf of the firm. Not a huge amount, just roughly €500m.
Merril Lynch €2.75m
The former Financial Regulator Liam O'Reilly was left red-faced after Merrill Lynch, whose board he sat on, was handed a then-record fine of almost €3m.
The IFSC-based firm was punished in October 2009 for two major trading incidents which saw the company lose over €306m due to several oversights, including a failure to supervise trader activity.
Mr O'Reilly sat as a non-exec director of the bank after joining the board in March 2007. He was first chief executive of the interim regulatory body the Irish Financial Services Regulatory Authority, and had served as assistant director general of the Central Bank of Ireland since 1998, leaving in 2006.
His appointment to the board of Merrill Lynch had sparked controversy at the time anyway, due to a perceived conflict of interests in working at an institution that he had once regulated.
Allied Irish Banks
State-owned AIB once again hit the headlines for all the wrong reasons in December 2010. The bank, bailed out by taxpayers to the tune of almost €21bn, was found to have overcharged over a quarter of a million of its customers.
The Central Bank found that the bank had breached the Consumer Protection Code in three areas over several years.
The 258,000 customers affected had been overcharged by an average of just under €100 each.
The Central Bank found that as well as this, there had been an unacceptable delay in notifying customers of the overcharging and in issuing refunds, and imposed a €2m penalty on the saved-by-the-taxpayer institution.
As well as being forced to pay out a record €3.5m this week, the bank was hit with a penalty of almost €2m in November of 2012 after it was found that it had not held enough cash in reserve to buffer it against potential risks.
How short was it?
Ulster Bank was obliged to hold €339m in capital, but was found to be €313m short of this in March 2013.
The bank said that, unlike its IT meltdown, no customers were affected, and parent company Royal Bank of Scotland immediately made a cash injection when the mistake was revealed.
Chief executive Jim Brown admitted the errors were "unacceptable" and pledged that the bank had "implemented a number of robust measures to ensure similar contraventions are not repeated".
Two separate fines of €1.225m
Although both fines were imposed at the same time, December 2012, the fact that they were levied on different subsidiaries of the Aviva group means that the insurer has the dubious honour of occupying both of the bottom spots on our 'top' 10 list.
Aviva Insurance Europe and Aviva Life and Pensions Ireland were both found to have failed to properly monitor stock lending carried out by Aviva investment managers on their behalf from 2005 to 2011.
The subsidiaries of the British insurance giant, which has the naming rights to one of the country's biggest stadiums, were not setting risk limits or receiving enough information from their risk management arm on the potential exposure of assets, leading the Central Bank to hit them with a joint fine of nearly €2.5m.
Sunday Indo Business