/etc/rcS.d

From the README file:
The scripts in this directory whose names begin with an 'S' are
executed once when booting the system, even when booting directly into single user mode.

The scripts are all symbolic links whose targets are located in
/etc/init.d/ .

To disable a script in this directory, rename it so that it begins
with a 'K' and run update-rc.d script defaults to update the order
using the script dependencies.