On New Year’s Eve 25 years ago, sane people worried that the modern world was about to melt down.

The millennium bug seemed to be threatening to crash the world’s computer systems, as technology struggled to distinguish between the years 1900 and 2000. The public, faced with daily predictions of potentially terrible outcomes, braced themselves nervously.

Dark jokes prevailed about avoiding being on “a life-support system at midnight on 31 December 1999”. In China, Zhao Be, then the head of the country’s millennium bug coordination efforts, commanded airline executives to be on a flight on 1 January 2000 to demonstrate any problems had been sorted.

Yet, in the end, nobody appeared to perish.

The same might be said of one of the earliest events to reveal the existence of the bug, which was also known as Y2K.

In 1987 Marks & Spencer received a batch of tinned meat that was rejected because the company’s computers thought it was almost 90 years past its January 2000 use-by date. Five years later, a Minnesota kindergarten invited one Mary Bandar to join its classes. She was 104 at the time.

These quirky errors were the result of early computer scientists solving a practical problem. To save space and speed up processing, computer dates were abbreviated. So January 1900 was 01/00 and December 1999 was 12/99. Trailblazing computer scientists had assumed that none of the computers would be in use by 1999. Only, it turned out there were so many legacy systems in operation that catastrophic results appeared a certainty to some.

One British family – the Perrons – abandoned their Wiltshire home in 1998 and headed to a hillside cottage near Forres, Moray, as they feared food shortages, the accidental discharge of nuclear weapons, the breakdown of global capitalism and rioting on the streets. Their new property had no running water or electricity but did possess its own generator, while water was available from a nearby stream and the family grew vegetables in their garden and ate eggs provided by their hens.

This level of preparedness would have given them full marks from Action 2000, a UK government taskforce created during the period to monitor computer readiness. With a budget of £17m, it would name and shame government departments and councils it judged were not ready – and the clamour to throw money at the problem grew and grew. The research firm Gartner estimated the total global cost of millennium bug remediation to be as high as $600bn (£475bn).

Yet, despite all of the effort and cost, issues still occurred.

Internationally, 15 nuclear reactors shut down; there were power cuts in Hawaii; and the bug was blamed for more than 150 pregnant women being given incorrect results of a Down’s syndrome test.

But the enduring story was how few problems actually occurred. The Perrons’ stash of stockpiled medicines remained untouched and life continued much as it had done before – prompting criticisms that the risks had been overstated. Some even threatened legal action as they suggested technology companies had exaggerated the likely effects of the bug – while communist Cuba dismissed the warnings as a capitalist conspiracy to boost spending on computers. These allegations were met with fury by the industry.

As Martyn Thomas, who led Deloitte Consulting’sY2K work internationally in the mid-1990s, argued in the Guardian in 2019: “The millennium bug was real and the internationally coordinated effort was a great success. Tens of thousands of failures were prevented. Some suppliers took advantage and sold unnecessary upgrades to their customers, but those of us who worked days, nights and weekends to meet the hard deadline of December 1999 are angered when ignorant people think that because we succeeded, the threat was not serious.”

Still, countries such as Italy, Russia and South Korea were said to have done little to prepare for Y2K, and yet appeared to experience no more technological problems than countries writing huge cheques. US spending was about double that of Italy’s, according to a weighted Y2K Spending Index compiled by the market intelligence group, International Data Corporation (IDC).

In the aftermath, Margaret Beckett, who led the UK government’s Y2K work, said: “It is not true that some of international partners ‘spent next to nothing’. We know from our many contacts in the International Year 2000 Co-operation Centre that a huge amount of work was done across the world, including Russia, Asia, Latin America and Africa.”

Or, as the trade publication Computer Weekly put it on 13 January 2000: “Two weeks into the new year, and the millennium bug is beginning to look like one of the greatest non-events in history. After dire predictions of global recession, nuclear meltdowns, and food shortages, 1 January was a serious anticlimax.”