This has happened twice now. I'd be using Safari and suddenly it locks up the entire system. It'll sit that way for at least a minute or two before recovering... for maybe another 20-30 seconds, then do it again. If I quit Safari, things return to normal. But if I start it back up, the issue resumes. The only fix is to reboot. This happened last night before I went to bed. I quit Safari and left it that way in case it just needed time. Came in this morning and restarted Safari. Immediately the problem returned. Everything was frozen solid. Dock was out, Activity Monitor was frozen, nothing at all responded. I ended up doing a hard reboot. But since then, it's been fine. The computer is a 2013 rMBP i5 2.6 with 16 gig of RAM. I'm just wondering if I'm the only one who has these problems. Seems to me that El Capitan is buggier than Lion, which is saying something.