The biggest mistake was made by Xerox and it happened in 1979. This cost them potentially up to $1.4 trillion. (The combined value of Apple, IBM and Microsoft).
That mistake cost them the chance to be one of the biggest company in the history of the world. That is a much bigger mistake than any of the others mentioned on these answers.
This is what happened:
Xerox PARC the company’s Silicon Valley R&D lab had created the working basis of the modern computer.
They made a computer called the Alto, it was the first computer that could do the following things:
The first computer that had a working Graphic User Interface, that means the first computer ever with a mouse and screen where you can move the cursor on things you click.
The first computer that was networked together, this was the first computer a email was sent on. (in the internal network). This basically became the basis of the internet.
It was also the first computer to use object orientated programming, the foundation for what all modern computer programs on your computer, phone and most electronic devices operate on.
What happened to this Alto computer? If they created all this in the Alto, why has nobody heard of it, why is nobody accessing Quora on a Xerox device?
It’s because Xerox made an 1979 investment in Apple, as part of that agreement they gave Steve Jobs access to their labs. Steve Jobs used this access to the first Alto to build the Lisa and eventually the first Mac.
Bill Gates who was writing Microsoft Office software for the Mac, had access to Apple’s source code. So he thought he would also make his own computer/ operating system. IBM made the hardware and he supplied the software.
Meanwhile nobody at Xerox HQ even acknowledged all this was happening, they failed the launch of the Alto and even though they created it, they never protected it or commercialised it.
Market Cap Of Each Company:
Xerox : $7B
Apple: $803B
Microsoft: $546B
IBM: $143B
Steve Jobs New Yorker Quote ““If Xerox had known what it had and had taken advantage of its real opportunities,” Jobs said, years later, “it could have been as big as I.B.M. plus Microsoft plus Xerox combined—and the largest high-technology company in the world.
Here is the mistake broken down:
Xerox senior management didn’t know their own R&D so they didn’t know its value.
Giving Steve Jobs access to their R&D.
When Adele Goldberg of the Alto R&D team at PARC labs refused to show Steve Jobs the technology, they ignored her advice and forced her to do it.
Not making the launch of the Alto a company priority.
Not getting the price of the Alto down. Apple for example re-designed their same mouse 20 times cheaper.