From LQWiki

Case Studies are real-world examples focussing on a particular group or company. For example you could be discussing buying a PC with Linux pre-installed and a case study would be System 76. The case study could discuss how much money they have saved by switching to Linux and how much more business they have received because of it.

Case Studies are quite often specifically chosen to show whatever product you are trying to sell in a good light. If you look at the Microsoft case studies, every single one mentions how much money they are saving by using MS and not Linux, or how much faster it is. Every single 'bad' case study will have been hidden and won't be mentioned. You can do case studies on how things went wrong and were then fixed, but big businesses usually just use them as a selling point and won't mention anything negative. This isn't done by lying on the case study - that would be too easy to disprove - but rather by showing the best ones and ignoring the worst.