While testing a build of Audacity, I noticed this confusing behavior:

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$ ./pre-inst-env guix build --no-grafts audacity
;;; note: source file /home/leo/work/guix/gnu/packages/audio.scm
;;;       newer than compiled /home/leo/work/guix/gnu/packages/audio.go
;;; note: source file /home/leo/work/guix/gnu/packages/libreoffice.scm
;;;       newer than compiled /home/leo/work/guix/gnu/packages/libreoffice.go
substitute: updating substitutes from 'https://privatemirror.example.com'... 100.0%
substitute: updating substitutes from 'https://berlin.guixsd.org'... 100.0%
substitute: updating substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
The following derivation will be built:
   /gnu/store/75rknckix6p6vslgf0fy1z8c6six0svg-audacity-2.2.2.drv
32.4 MB will be downloaded:
   /gnu/store/xab948cliy5b9qqkkh3f3v2ffjgg5jgl-wxwidgets-3.0.4
   /gnu/store/rgmb5s24bmwb9lfd6y6vyqbcf872ksq5-suil-0.10.0
   /gnu/store/df5w7bx13yijdrrccf0jh8733zsr9gff-qtbase-5.11.1
substituting /gnu/store/xab948cliy5b9qqkkh3f3v2ffjgg5jgl-wxwidgets-3.0.4...
downloading from https://privatemirror.example.com/guix/nar/gzip/xab948cliy5b9qqkkh3f3v2ffjgg5jgl-wxwidgets-3.0.4...
 wxwidgets-3.0.4  10.3MiB                            1.2MiB/s 00:09 [##################] 100.0%
gets-3.0.4  10.3MiB                            1.0MiB/s  wxwidgets-3.0.4  10.3MiB                            1.1MiB/s  wxwidgets-3.0.4  10.3MiB                            1.1MiB/s  wxwidgets-3substituting /gnu/store/37ldf3jfbq8qv0i0bpfrbbjd4ska605l-mariadb-10.1.35...                            1.1MiB/s  wxwidgets-3.0.4  10.3MiB                            1.1MiB/s  wxwidgets-3.0.4  downloading from https://privatemirror.example.com/guix/nar/gzip/37ldf3jfbq8qv0i0bpfrbbjd4ska605l-mariadb-10.1.35...widgets-3.0.4  10.3MiB                            1.1MiB/s  wxwidgets-3.0.4  10.3Mi mariadb-10.1.35  60.6MiB                            973KiB/s 00:20 [#####             ]  31.7%^C
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It needed to download mariadb, but this wasn't included in the report of
what would be done.

For later reference, this happened on commit
b0f43001195c69f730b49b0bf9de516edd53baed.

It's on Debian, with the guix-daemon from
f9051e387056fb1c29d8e67b76754168b304449d (0.15.0-4.7fed935).