On July 14, a series of errata were posted for the two supported releases, 5.8 and 5.9, to address reliability and security issues.
The announcement:
http://marc.info/?l=openbsd-announce...4382106375&w=2
In a follow-on announcement, the Project determined it would remove the kern.usermount option for 6.0, as panics can be caused by certain variations of the mount(2) syscall.
http://marc.info/?l=openbsd-announce...4517406640&w=2
I'm certain this will appear in the 6.0 Upgrade Guide, but wanted to bring this to the attention of our user community here, as several people are mounting via kern.usermount. The simplest alternative may be to use doas(1).
Those interested in the technical details of the syscall issues may refer to the discussion on the tech@ mailing list. There have been several threads, the most recent being this one:
http://marc.info/?t=146826849800004&r=1&w=2