This is probably going to be a short post, as I'm not feeling too well, but some recent stuff at my job has had me thinking of rules by exception a lot lately. See, whenever you are making rules you need to be careful, because a rule doesn't just say what it says, it says what it doesn't say too, and often that can be a very large area. Whats worse is that if a rule isn't written clearly, there is a large area of confusion as to what the rule does and doesn't mean. When making a game it is your primary job to minimize these areas, and make a system that is easy to understand.
So, what exactly do I mean by a rule of exception? Well, lets give an example then from the real world. You are driving your car downtown and see a sign posted that says "No Parking Between 8 and 5". Pretty straight forward right? You can't park there between those hours. But it also means that you can park there outside of those hours. See, because there is a rule making a special condition for those times, everything not covered by those times must then be fair game for whatever is being prohibited.
Now, in a game book this gets a bit trickier. See, the whole thing is a big collection of rules, and rules of exception can apply in lots of places. Say for example your class lets you "take any one advantage, ignoring all pre-requisites". That is pretty handy, but now another class only says "Take any one advantage". There are two options here, one is that because the special ability to ignore exceptions is clearly stated in one place, and not another, that it is intentional and this is a weaker version of the same ability. Alternatively, it could just be an editing mistake, and they are supposed to be the same ability.
Some other fun ones might be things like "A called shot to the head does double damage" with no special text for other called shots. Does that mean only to the head gets a special effect? Or does it mean they all do?
These are problems that come up because a rule that specifies a specific situation, by making an exception, rules that everything that isn't that situation must fall under something else - usually the opposite. This gets even more complicated when making a game because there is always the possibility of it being a simple mix up, or that the two things are supposed to be completely separate and not related.
This is also why simple, straight forward rules are the best. You really want to stick as close to the KISS concept as possible, because with every complication you are furthering the grey area of confusion, as well as the list of unspoken rules. That confusion can cause debates and rules discussions, and when people are arguing over how your game is supposed to work, they're not playing it or having fun.
This is also true of everything. When you put something down as a rule, you also put down the opposite of it down as a rule. If you say I can't whistle between 3 and 4, then I can whistle when it is not between 3 and 4. Try to keep that in mind next time you are making, reading, or adding rules to something. It may help out.