Paying loads of money for the rights to make ET, setting it up as the big holiday seller then giving your programming team just 6 weeks to make it.
CD-i games. Just all of them.
Not making an outcast 2 
The huge amount of failed MMORPG's that are out there....it's like a dot com burst.
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A few counter claims.
800XL was a great computer by Atari, I owed one and loved it to death.
Rare lost alot of it's talented people when it was sold to Microsoft. The Rare that nintendo sold (49%) isn't the same as the one Microsoft bought. If Nintendo had bought rare, it might of been a very different company to what it is now, but thats something we will never know.
"..just keep on trying 'till you run out of cake"
