See also https://reproducible-builds.org/
and https://github.com/bmwiedemann/theunreproduciblepackage/tree/master/race

When packages build with make -j$N   (N>1),
it sometimes results in binaries that differ between builds
and thus trigger rebuilds of depending packages
and are published to mirrors and users
when actually nothing really changed.

Build results can also be incorrect due to races.

Often it is possible to fix races by adding proper dependencies between build
targets (e.g. https://bugzilla.sudo.ws/show_bug.cgi?id=842 )
or fixing involved racy tools