Chronos Tachyon writes in http://bugs.debian.org/366269

The select_shell() function in /usr/bin/updatedb has a bug in the way it calls /bin/su; specifically, it attempts 'su "$LOCALUSER" -s $SHELL false', which runs '/bin/bash false', thus attempting to execute /bin/false as a shellscript.  (The exit code 126 and error message 'cannot execute binary file' are ignored entirely by updatedb.)

The correct usage of su would be to add "-c" after "-s $SHELL" on lines 87 and 91.  This is important if, for instance, the administrator has set $LOCALUSER's shell to /bin/false (which is how I discovered this bug).

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Older versions of su as found on Debian (e.g. in sarge) accepted this syntax, but this is consideerd a bug.
cu andreas