In the world of IT, even small mistakes can spiral into massive problems. A single line of bad code, a rushed update or a missed warning sign can cost millions and cause considerable damage. Looking back at some of the biggest failures in tech history is a great way to understand what really matters in IT so here’s ten of the biggest mistakes ever made in the world of tech and what they can teach us.

1. The Mars Climate Orbiter (1999)

In 1999 NASA lost a $125 million spacecraft because one team used metric units while another used imperial. The mismatch caused the orbiter to enter the atmosphere at the wrong angle and burn up. It’s one of the most expensive mistakes ever and shows how critical communication and clear standardisation is when it comes to complex systems.

2. Ariane 5 Rocket Explosion (1996)

The European Space Agency’s Ariane 5 rocket self-destructed just 40 seconds after its launch in 1996 due to a software overflow error. Code from an older rocket was reused without proper testing. The failure cost around £300 million and shows how dangerous it can be to reuse old legacy systems without properly reviewing them.

3. AT&T Network Collapse (1990)

A faulty software update in 1990 caused AT&T’s long distance network to crash, blocking around 75 million calls across the US. The issue came from a single line of code that triggered a chain reaction. It proved how fragile large systems can be without staged testing and rollback plans.

4. Therac-25 Radiation Machine (1985 to 1987)

In the mid-80s, the Therac-25 Radiation Machine delivered massive radiation overdoses due to software errors, leading to several patient deaths. The system had removed hardware safety checks and relied entirely on software, running for years before the error was noticed. It’s a chilling example of why safety critical systems must always have multiple layers of protection.

5. Intel Floating Point Bug (1994)

Intel shipped millions of Pentium processors with a flaw that caused rare but serious calculation errors. At first Intel dismissed the issue, but public backlash forced a recall in 1994 that cost around $475 million at the time (more than £750 million in todays money). It shows how ignoring user reports can turn small bugs into huge reputational damage.

6. The Patriot Missile Failure (1991)

During the Gulf War, a US Patriot missile system failed to track an incoming Scud missile due to a rounding error in its internal clock. The result was 28 soldiers killed. The system had been running continuously for too long without a reboot, proving that simply turning your tech off and on again more often could be critical.

7. eBay’s 22 Hour Outage (1999)

eBay went offline for nearly a full day in the late-90s due to database corruption during an upgrade. The outage cost millions in lost sales and customer confidence. It highlighted how live system changes without full testing can cripple even the biggest platforms.

8. London Stock Exchange Crash (2018)

A software bug in a new trading system caused widespread outages to London Stock Exchange in 2018, forcing traders to revert to paper-based processes. The disruption lasted hours and exposed how even the biggest financial institutions still struggle with poorly managed upgrades.

9. The Boeing 737 MAX Software (2018 to 2019)

Flight control software in the new Boeing 737 Max, which removed control from the pilots without warning in certain emergencies, lead to two fatal crashes in the late-2010s. Pilots weren’t informed at all about the new system and received no substantial training in the new plains as Boeing deliberately hid any major upgrades they’d made, making it seem far more similar to the previous model than it actually was; They did this so that the plains could be rolled out faster without the need for crucial piolet training, falsely claiming experience with the previous model was enough. It’s one of the most serious examples of software failure in modern history and a reminder that transparency and propper testing save lives.

10. Google Docs Bug (2017)

A faulty update in 2017 locked thousands of users out of their own Google docs, wrongly flagging them as violating terms. The issue was quickly fixed, but it showed how automation without proper checks can block legitimate access to critical data.

What We Can Learn

Across all these failures, the same themes keep appearing. Poor testing, rushed changes, lack of backups and over reliance on software without safeguards. Most of these disasters weren’t caused by hackers or complex attacks, they were caused by basic human error.

For businesses today, the lessons are simple but vital. Keep systems updated, test changes properly, maintain backups and don’t ignore warning signs. Proactive IT management isn’t a luxury, it’s essential.