I’ve noticed 3 “missed schedule” posts since upgrading to a version of WordPress 4.0. I believe 2 were while I was running release candidates, but I know 1 was when running the real version. I think all 3 missed were when multiple posts were scheduled at the same time.
I have the most information about the 3rd missed post because I still haven’t corrected it. It was on real WordPress 4.0 and schedule at the same time as 2 other posts. It was the last of the 3 scheduled for that time to be created. I don’t think it’s a normal cron error because that wouldn’t explain why some posts scheduled at the same time posted while 1 didn’t.