So it sounds like I have less "metagame" than your games and more "metagaming" by the "classic example" as you call it, but even so, it has no impact on achieving the goals of play whatsoever. Which to me argues for not thinking about it so much.
What if playing the part of the character (and avoiding player knowledge the character wouldn't have) when dealing with challenges, is a key part of the goal of playing?
Moldvay in his Basic book notes that "A player should not allow his or her character to act on information that character has no way of knowing (for example, attacking an NPC because the NPC killed a previous character run by the player, even though the NPC and current character have never met). If the players get careless about this the DM should remind them. The DM may, in addition, forbid certain actions to the characters involved. The DM should make it clear to the players before the adventure begins that characters may not act on information they don't have. It will save lots of time later."
Gygax in the 1e DMG talks about "players that continually attempt actions or activities their characters would have no knowledge of" among those that are troublesome and need to be dealt with. (I'm not sure his suggestion about using peer pressure to do it is necessarily the way to go....).
Zeb Cook & co. in the 2e PhB notes that "The player makes deicsions, interacts with other charactersand players, and, essentially, "pretends" to be his character during the course of the game." In the DMG they go even further:
"Your players must understand the distinction between what they know as players and what their characters know. Your players have read the rules and shared stories about each other's games. They've torn out their hair as the entire party of adventurers was turned into lawn ornaments by the medusa who lived beyond the black gateway. That is all player information. No other characters know what happened to that group, except this: they went through the black gateway and never returned.
The problem of player knowledge/character knowledge is always present, but it is much worse when players control more than one character in the same region. It takes good players to ignore information their characters have no way of knowing, especially if it concerns something dangerous. The best solution is to avoid the situation. If it comes up and players seem to be taking advantage of knowledge they shouldn't have, you can discourage them by changing things a bit. Still, prevention is the best cure."
Aldridge in D&D and Philosophy notes "However most D&D players would suppose that characters should not act on knowledge that would be unavailable to them according tot the usual causal laws (physical, magical, or otherwise) established within the particular setting." He notes that the DMG II for 3.5 also says that players shouldn't act on information their characters don't have.
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Of course lots of rules and guidelines are ignored, modified, and hacked by players, and rule 0 gives that a ringing endorsement. So the importance of the rulebooks' condemning on the use of out of character knowledge are no more binding than any of the other things everyone house rules away.
Rose in D&D and Philosophy brings up the a gunpowder creating example like the one used posts ago to lead into a discussion of ludology vs. storytelling in metagaming. The author comes to many of the same conclusions offered previously for the players "antics" in trying to make gun-powder contra to the story. In real life the DM angered the player greatly by resorting to fiat, where a simple badly failed experiment might have given fine in-game cover.
Aldridge in the same book talks about the immersion-gamer spectrum, and goes into how the player and character relate in the ideal play Gygax would use peer pressure to enforce, and whether it matters that it may very well be impossible to do.
It feels to me like avoiding using player knowledge the character has is more true to the spirit D&D's creators meant it to have... but D&D's creators aren't playing at your table, and even if they were the solution of just dealing with whatever the player says in game might be easiest all around.