Another buildworld crash, but this time with a twist:

Rather than start buildworld from scratch, the process was started with
make -j2 kernel-toolchain
then followed with 
make -j4 -DNO_CLEAN buildworld

There were a few obvious intervals of difficulty during both kernel-
toolchain and buildworld, but toolchain finished successfully and
buildworld managed to get through at least one session of trouble
with swread and pfault status in the top window.

Eventually the console reported "segmentation fault" with no
indication what process triggered the fault, then began to emit 
(da0a is /var, da0d is /usr):

g_vfs_done():da0d[WRITE(offset=52945190912, length=131072)]error = 5
g_vfs_done():da0d[WRITE(offset=52945321984, length=131072)]error = 5
g_vfs_done():da0a[WRITE(offset=65536, length=4096)]error = 5
g_vfs_done():da0a[WRITE(offset=8585216, length=4096)]error = 5
g_vfs_done():da0a[WRITE(offset=268632064, length=32768)]error = 5
g_vfs_done():da0a[WRITE(offset=277078016, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=65536, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=20709376, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=14447476736, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=24396693504, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=43330764800, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=51864764416, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=51865321472, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=52520681472, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=52945059840, length=131072)]error = 5
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 3 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 2 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 1 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 0 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Error 5, Retries exhausted
g_vfs_done():da0d[WRITE(offset=43892801536, length=20480)]error = 5
g_vfs_done():da0d[WRITE(offset=51887759360, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=52917723136, length=8192)]error = 5
g_vfs_done():da0d[WRITE(offset=52944699392, length=98304)]error = 5
g_vfs_done():da0d[WRITE(offset=52945059840, length=32768)]error = 5
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 80 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 3 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 80 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 2 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 80 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 1 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 80 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 0 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 80 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Error 5, Retries exhausted
g_vfs_done():da0d[WRITE(offset=52945190912, length=131072)]error = 5
g_vfs_done():da0d[WRITE(offset=52945321984, length=131072)]error = 5
g_vfs_done():da0a[WRITE(offset=65536, length=4096)]error = 5
g_vfs_done():da0a[WRITE(offset=8585216, length=4096)]error = 5
g_vfs_done():da0a[WRITE(offset=268632064, length=32768)]error = 5
g_vfs_done():da0a[WRITE(offset=277078016, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=65536, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=20709376, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=14447476736, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=24396693504, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=43330764800, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=51864764416, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=51865321472, length=32768)]error = 5
g_vfs_done():da0d[WRITE(offset=52945059840, length=131072)]error = 5
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 3 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 2 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 1 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Retrying command, 0 more tries remain
(da0:umass-sim0:0:0:0): WRITE(10). CDB: 2a 00 06 69 e2 c0 00 00 40 00 
(da0:umass-sim0:0:0:0): CAM status: CCB request completed with an error
(da0:umass-sim0:0:0:0): Error 5, Retries exhausted
g_vfs_done():da0d[WRITE(offset=43892801536, length=20480)]error = 5
g_vfs_done():da0d[WRITE(offset=51887759360, length=4096)]error = 5
g_vfs_done():da0d[WRITE(offset=52917723136, length=8192)]error = 5
g_vfs_done():da0d[WRITE(offset=52944699392, length=98304)]error = 5
g_vfs_done():da0d[WRITE(offset=52945059840, length=32768)]error = 5

Meanwhile top reported essentially 100% idle time. Issuing 
shutdown -r now
on a root shell eventually forced a reboot, the system came up
without visible ill effect after typically-long fsck session. 

The buildworld log reported that it was building libraries at the
time of the crash, so I've simply restarted the original buildworld
to see if the system can pick up where it left off.