Nintendo:
Virtual Boy
Nintendo 64 using carts
Bad 3rd party relations
Not pushing eye candy/HD on Revolution. That's really going to hurt them in 2010 when a lot of gamers will have HDTVs.
Not pushing DS's technology more. They're getting great sales now, but when PSPs price comes down and equals it, it could hurt it later.
Ignoring the online market.
They have ADD with their userbase target. First they want to get the adult market with Eternal Darkness, Resident Evil, but then they go for "Mario Sunshine", purple gamecubes, and cartoony zelda. Great games...but completely contradicts their new target.
Microsoft:
Being anti-keyboard/mouse. FPS + gamepad = SUCK. Not to mention we could have Warcraft 4 on Xbox 360.
Their Xbox 1 was a complete financial failure. But then again, MS may have only been concerned about market penetration, which was a moderate success.
Sony:
Somewhat negligant of making hardware developer friendly
2 controller ports on PS2
Sega:
Everything between, but not including, genesis and Dreamcast.