RROD
Buying Rare/mismanagement of Rare
Other buyouts that don't make much sense. I'm sure are plenty but I can't think of most off hand. I didn't agree with Skype but w/e.
BING but in the long run could be profitable but that remains to be seen.
Having their own cell phones then killing it shortly after. Lots of wasted money in R&D.
Killing off Ensemble Studios (Halo Wars, Age of Empires/Mythology...)
Lots of people forget about Ensemble Studios.
It's just that simple.