Off the top of my head:
1- Nintendo backstabbing Sony during PS1 development- Sony then finished Nintendo's dominance of the industry and destroyed them for a decade. Of course, Nintendo are back on top, but that's another story for another day....
2- RED RING OF DEATH- People already hated Microsoft and this just ruined any chance MS had for winning this gen. They used to have a big lead on Sony but this put off potentially millions of customers. The phrase "RROD" is now synonymous with the Xbox brand, people will never forget the RROD.
3- FIVE HUNDRED AND NINETY NINE U.S. DOLLARS (or £425)- Ah, only Sony could price a console this high and still have it do well. But if it was $450-500 at launch, the gap between PS3 and 360 would be so much smaller, perhaps it could have already passed the 360.
4- Sega not using a DVD drive with the Dreamcast- as the PS2 shows, this would have been a good idea. I'm not too familiar with the history of Sega's mistakes but I know they somehow pissed off a lot of people with the Saturn. And this made people reluctant to purchase the Dreamcast.
