If you ever want to have a lot of fun, I recommend that you go off and program
an imbedded system.  The salient characteristic of an imbedded system is that
it cannot be allowed to get into a state from which only direct intervention
will suffice to remove it.  An imbedded system can't permanently trust anything
it hears from the outside world.  It must sniff around, adapt, consider, sniff
around, and adapt again.  I'm not talking about ordinary modular programming
carefulness here.  No.  Programming an imbedded system calls for undiluted
raging maniacal paranoia.  For example, our ethernet front ends need to know
what network number they are on so that they can address and route PUPs
properly.  How do you find out what your network number is?  Easy, you ask a
gateway.  Gateways are required by definition to know their correct network
numbers.  Once you've got your network number, you start using it and before
you can blink you've got it wired into fifteen different sockets spread all
over creation.  Now what happens when the panic-stricken operator realizes he
was running the wrong version of the gateway which was giving out the wrong
network number?  Never supposed to happen.  Tough.  Supposing that your
software discovers that the gateway is now giving out a different network
number than before, what's it supposed to do about it?  This is not discussed
in the protocol document.  Never supposed to happen.  Tough.  I think you 
get my drift.