I use fsck_ffs -y weekly, most of the time after Xorg crashes, sometimes to fix the filesystem on a thumbdrive that has "stuck" ( one-line errors ( such as "could not read superblock" ) mostly are unexpectedly fixed by that command... maybe it is just luck or the same situation just happening repeatedly more often than other less fixable ones.) The point being, I should have known about it years ago, could have saved time and data.