This is a regression since [[ https://reviews.freebsd.org/D34741 | D34741 ]], which causes bogus entry to be generated in some cases. It's quite a dangerous since if the output of the make command is fed to a dump removal script it might kill the system.
> Author: Maksym Sobolyev <sobomax@sippysoft.com>
> Date: Mon Aug 25 10:16:41 2025 -0700
>
> Fix list-old-dirs to not generate empty line at the end of the
> output if OLD_DIRS happens to have a space at the end of the
> last word of the output before the final \n. Then that space
> is turned into \n and we end up with a blank line.
>
> The make -V OLD_DIRS looks like so:
> 0000ce60 73 72 2f 74 65 73 74 73 2f 67 6e 75 2f 75 73 72 |sr/tests/gnu/usr|
> 0000ce70 2e 62 69 6e 20 75 73 72 2f 74 65 73 74 73 2f 67 |.bin usr/tests/g|
> 0000ce80 6e 75 2f 75 73 72 2e 62 69 6e 2f 64 69 66 66 20 |nu/usr.bin/diff |
> 0000ce90 75 73 72 2f 74 65 73 74 73 2f 67 6e 75 2f 6c 69 |usr/tests/gnu/li|
> 0000cea0 62 20 0a |b .|
> 0000cea3
> Futhermore this gets converted into a "/" i.e. root fs when
> calling check-old-dirs.
> $ make check-old-dirs
> ...
> /lib/casper
> /usr/lib/debug/lib/casper
> /etc/skel
> /etc/casper
> /
> /usr/lib/debug/
> $
>
> This is the regression since a8267ecc3df0a.