I have kernel panics on 10.3 on a regular basis, varying from several times per day to a couple of times per week according to workload.

There's something inside wait6 and kevent that causes a thread to sleep whilst holding a non-sleepable lock. I hit it more than most I suspect because on my systems a lot of processes are blocked in kqread, whereas with the stock toolset one sees few processes using kevent.

I brought the subject up last year on freebsd-hackers, but nothing came of it.

* https://news.ycombinator.com/item?id=13622945