Sure; the list of startup services is maintained there, for one (in one of a number of ways), but a variety of programs allow the administrator to configure hoo

Sure; the list of startup services is maintained there, for one (in one of a number of ways), but a variety of programs allow the administrator to configure hooks to execute when something happens as well. Then there are lists of variables that are in fact shell scripts, names of users’ configured shells, mount options, kernel modules, ... . Arbitrary code execution immediately (as opposed to after next reboot or at least a service’s configuration being reloaded) seems harder, but I expect it to be possible as well. ETA: Took a couple of minutes, huh. A sibling comment mentions the dynamic linker, which apparently maintains a cache of well-known library locations in /etc. This yields execution as soon as a new dynamically linked executable is started, which probably qualifies as immediate. (More care would be required to avoid breaking the system in the process, though.)