Hi,

I was updating Guix today with the option rounds=2 and I hit an error 
with mpv having different outputs everytime it was built.

The message is this:
building /gnu/store/qkmylc2cd950kf4zk8y3ay0hpwcvjpvr-mpv-0.32.0.drv...
\output ‘/gnu/store/q71zaxglbwmw3z4r1a3jmf1mgwrpmv4c-mpv-0.32.0’ of 
‘/gnu/store/qkmylc2cd950kf4zk8y3ay0hpwcvjpvr-mpv-0.32.0.drv’ 
differs from previous round
build of /gnu/store/qkmylc2cd950kf4zk8y3ay0hpwcvjpvr-mpv-0.32.0.drv 
failed
View build log at 
'/var/log/guix/drvs/qk/mylc2cd950kf4zk8y3ay0hpwcvjpvr-mpv-0.32.0.drv.bz2'.
cannot build derivation 
`/gnu/store/3j9pkj5kad042i1zyqzdw2vl1yfsbznk-celluloid-0.19.drv': 1 
dependencies couldn't be built
guix upgrade: error: build of 
`/gnu/store/3j9pkj5kad042i1zyqzdw2vl1yfsbznk-celluloid-0.19.drv' failed

And my system is this:
Generation 9	Jun 23 2020 19:41:06	(current)
  guix aa246ea
    repository URL: <https://git.savannah.gnu.org/git/guix.git>
    branch: master
    commit: aa246eaa6b338fc6509bd5e6b93fa23121b2c95c

Let me know if there is more information I can provide.