1. Turning down JK Rowling
Twelve publishing houses rejected JK Rowling's Harry Potter manuscript before Bloomsbury finally took her on following the advice of the company chairman's eight-year-old daughter Alice.
The books were subsequently translated into over 60 languages and have earned Rowling a reported $1bn (£670m).
2. Throwing away that Bitcoin portfolio
James Howells bought 7,500 Bitcoins in 2009 when their value was next to nothing. By 2013, one Bitcoin was worth £613, giving the Welshman a portfolio worth £4.5m. The only trouble? He'd left his hard drive tucked away in a drawer for years and then thrown it away without a minute's thought. After realising his mistake, he made a hopeful trip to his local landfill site where he was told the hard drive could be at any spot under around 5ft of rubbish.
3. Not buying Google for $1m
Google founders Larry Page and Sergey Brin approached Excite CEO George Bell in 1999, saying they were looking to sell the search engine for around $1m. With Bell not keen on the initial offering, the pair went down to $750,000 in a bid to tempt him. He still rejected. Today, Google is valued at around $365bn. Oops.
4. Not shooting Hitler
In 1914, British soldier Henry Tandey, who went on to become the most decorated private in the First World War, came across an injured and unarmed Lance Corporal Adolf Hitler in a ditch, but reportedly decided not to shoot him in cold blood (although there is some dispute over the accuracy of this story).
5. Selling 610,000 shares instead of one
In 2005, a Japanese trader cost his company £190m after a so-called "fat finger" trade in which he sold 610,000 shares for 1 yen (0.5p) instead of selling one share at 610,000 yen as he was supposed to.
Despite repeated requests from Mizuho Securities to cancel the trade, the Tokyo Stock Exchange refused to comply and the company was forced to buy back the shares at an inflated cost.
6. Angering Genghis Khan
Genghis Khan, the ruler of the Mongol empire had sought to open diplomatic and trade links with Ala ad-Din Muhammad, Shah of the neighbouring Khwarezmid empire (modern day Iraq/Iran) in the 13th century.
However, after the offer was rejected and a Mongol diplomat beheaded, Khan reacted furiously, sending in an estimated 200,000 warriors and utterly destroying the empire.
7. Turning down Brian Acton and Jan Koum for a job
Facebook turned down programmers Brian Acton and Jan Koum at job interviews in 2009.
A few years later, Facebook paid $19bn (£11.4bn) for WhatsApp - the company the pair had developed after being rejected.
8. Ordering trains that were too wide
The French state railway SNCF spent $15bn on a new fleet of trains this year. Unfortunately, they were too wide for 1,300 station platforms across the country; a problem that will cost and estimated €50m (£36m) to fix.
“It’s like ordering a big, new car without checking the width of your garage,” said Emmanuel Grondein, of the SUD-Rail trade union.
9. Signing Brian Poole and the Tremeloes
In 1962, record label Decca were looking to sign an up and coming band. They auditioned two young bands at their studios in London, deciding to sign Brian Poole and the Tremeloes. The one they rejected? A four-piece outfit from Liverpool known as The Beatles.
10. Misspelling a company name
The British government was sued for £9m after a clerical error inserting a rogue "s" saw the wrong company recorded as being in liquidation.
More than 250 people lost their jobs when Companies House mistook a 124-year-old Welsh family business called Taylor and Sons for Taylor and Son - a company that filed for bankruptcy in 2009.