Recently, I had the pleasure of cleaning up a failed Exchange 2013 install in a 2012 R2 AD. The client had attempted to install Exchange 2013 on his own. When the install failed, instead of figuring out why, he simply tried to reinstall Exchange on a different server. Not a good idea. The failed install left traces all over the AD, which could be cleaned up without too much issue if that had been the extent of the damage. But, with the addition of a second failed install, not only was it impossible to install Exchange, it pretty much hosed up the AD itself. This required a complete and total reinstall of AD, after which Exchange installed without issue. The long and short of all this is that a failed Exchange install MUST be fixed to proceed!