(crap, first reply was eaten... trying again!)
Thanks for replying, RC!
Again, the "reasonable person" standard.
If it is impossible to do the same stunt twice in the same encounter, a reasonable person is likely to have noticed this. If the PCs make decisions based upon a knowledge of how the world works, it is just as reasonable that the NPCs do.
PCs and NPCs don't make decisions, only players do! We can decide what they know and what they don't know. We can decide that they have an Order of the Stick-level of knowledge about the game mechanics. We cna also decide that they treat the world just as we treat our own.
Again, why would you decide that characters know the game rules - unless you want them to, and if that creates a problem for you, why would you want them to?
Or would you argue that the PCs shouldn't know the game rules? "Sorry, Joe, but your fighter doesn't know that he can use his Kewl Powerz only once a day. Really, you should be trying to use your best ability this round. And next round. And next round......."
I would not tell another player how to play his PC. I'd say, "Look, Joe, this 4e thing is messing with my brane. Your fighter doesn't know he can only use Comeback Strike once per day. Tell me
why he doesn't use it this round."
"Simple - he can only use it when he gets
really pissed off. And he's not that pissed off."
"Who says?"
"I do. He has no control over his unquenchable rage - only I do."
"I see. He's kinda like a powderkeg and you're the match. I see why you named him Mr. Furious. But that still doesn't do it for me. What if you decide he gets
really pissed off, but he's already used his Daily power?"
"Then I spend an Action point and duplicate the effect."
"And if you don't have one?"
"Then I describe him getting really pissed off, and if anyone misses him I describe it as a hit! Or, he's already pushed to his limit and he get smacked around anyway - no juice left in the tank. Really, let's just play and see if there's a problem in what I describe."