The title states the issue. If you put an invalid entry in fstab but put the nofail option in the mount options, busybox mount -a and umount -a seem to disregard nofail. I grepped through busybox code and could not find a reference to nofail in the mount/umount code, so I think it isn't supported.
Upstream issue filed: https://bugs.busybox.net/show_bug.cgi?id=8551
(In reply to Austin English from comment #1) > Upstream issue filed: > https://bugs.busybox.net/show_bug.cgi?id=8551 Fixed upstream now.