many packages FTBFS, if $TAPE is set
Many packages FTBFS (silently!) if an environment variable TAPE
is set. That happens if your rules script uses something like
tar -c modules | bzip2 -9 > omfs.tar.bz2
for example. If $TAPE is set, then tar writes to $TAPE instead
of stdout (possibly corrupting the tape you had inserted). bzip
succeeds (regardless whether a tape is in), rules succeeds, but
your *.deb file is bad.
I am sure you agree that this is a fatal failure in the package.
It should be
tar cf - modules | bzip2 -9 > omfs.tar.bz2
But most people simply don't know that their rules file corrupts
tapes. First thing would be to detect these packages. This could
be done in the autobuild procedure done on the debian hosts, e.g.
by setting $TAPE to point to some kind of "watchdog file". If the
build succeeded, but the watchdog file contains some data, then
obviously something went wrong.