6. Allowing Sony to make the Playstation.
Who did it: Nintendo
Cartridges were Nintendo's bread and butter for years.
The NES, the Game Boy, the SNES-cartridges were here to stay. Until they weren't. Knowing that pesky CD's were on the horizon, Nintendo went to Sony to create a new CD-based console for them. Everything was going well until Nintendo decided that, nah, they weren't going to make a CD-based console after all and were going to stick with cartridges. Well, you don't just shun a powerful company like Sony and expect nothing to happen.
Sony decided to make their own CD based console, which would eventually become the Playstation. And we all know what happened after that. Nintendo, in cutting ties with Sony, ultimatley created one of their worst enemies. In the end, Nintendo ultimately made a CD-based console with the Gamecube, but it was too little, too late. Fortunately for Nintendo, the Wii was a boon for them. Hopefully, the Wii U can find its footing.