Hmmm.... for example, a player who thinks one or more of the skill costs for his profession is unreasonably high. Has a good, reality based, logical argument to support it, too. And he's not a power gamer, he's not trying to break your game. He's a character concept guy who's griping because his skill costs seem "unrealistic".
But if changing his skill costs makes that one profession so powerful relative to the others that no one else wants to play anything else, your game is crippled until you undo the damage done by that ruling.
Example 2: Green-as-grass GM, looks at the system, decides that it's too hard to survive low levels and that treasure is too hard to get, it doesn't look like any fun. Solves the problem quick and easy by making magic items both more common and more powerful. Works well too, very heroic, very high power, high fantasy game.... for about a year, maybe two. Now he has PCs so powerful that nothing short of the Gods can even challenge them, much less defeat them. Once you've whupped butt on the Gods, what's left? The entire game universe is dead, there's no practical solution except to start over.
See what I mean? I suspect more damage is done by GMs and players not thinking things through far enough than by intent. So yeah, rules additions should be as bulletproof as you can get em. Just like combat experience, playtesting the consequences of a rule is something you get just after you need it.
