"dpkg-buildpackage -j" looks similar to "make -j", and therefore
it happens frequently that people pass -j instead of -J to
A variant of this problem is #905788, with "sbuild -j5" passing
-j5 to dpkg-buildpackage (and sbuild not even supporting -J).
In this specific case the software itself does actually support
parallel building and uses "dh --parallel", but several
dh_auto_build calls end up being run in parallel with -j.
And build failures are not even the worst problem, we do have
plenty of packages in the archive that successfully build a
package with some contents missing when a part of the build fails.
It saddens me when I think about how much time I have already
wasted debugging problems that turned out to be caused by
"dpkg-buildpackage -j", and I do not see any usecase where
using -j would actually be a good idea.
dpkg-buildpackage should abort immediately with an error
message when -j gets passed.