Just today (09/05/2016) morning, my El Capitan 10.11.5 has failed to start. After the chime sound, it stays at black screen forever. I am about to reinstall it, and recover from Time Machine backup. I don't want to persuade myself to believe "Mac OS X is stable" anymore. I know 10.11.5 is still in beta. However, I have encountered about 10 times of freezes/kernel panics in last month, starting from 10.11.4. I don't know why. And, I must say I am lucky because I just made a Time Machine backup yesterday. Does anyone encounters so many time of kernel panics in a month? Update: problem solved. The culprit is the internal SSD. With that faulty SSD inside, Apple staff cannot even run diagnostics on Mac. And they can run it after removing that SSD. Basically I need to pay $625 to replace that SSD. But luckily Apple has a quality program to replace faulty SSD, and mine is eligible. So, after swapping a new SSD inside, I can use my Mac as usual again. Solving this issue just takes a whole day for me, which is today. (11/05/2016) I am happy with this result.