This came up last night and, quite frankly, annoyed the heck out of me. We were in combat with a group of trolls (we're about 2/3 through of King of the Trollhaunt Warrens) and our wizard cast Acid Arrow on a troll; she rolls and misses, and was about to end her turn when I reminded her that it does half damage on a miss; the DM points out that this is metagaming since my Dragonborn Fighter wouldn't know that (which is nonsense anyways, since I've seen her cast the spell before). I argue that it's not metagaming to point out the rules of the game to a player, since the character would know their own powers (the player is a little forgetful). DM says no they wouldn't, and if you forget something then oh well, your character forgets it too.
So, if the character forgets that acid burns, it doesn't happen?
Apparently your campaign is set in the world of Looney Tunes. Tell your DM that your next character is going to be an absent-minded fighter who constantly forgets that wounds cause him to lose hit points. Alternatively, run off cliffs and demand Perception checks to see if you notice the lack of ground.
On a slightly more serious note: Your DM may be using this "metagaming" business in a poor attempt to express a different objection - perhaps he feels there's too much kibbitzing and retconning going on in combat, or that the knowledgeable players are "crowding" the less rules-savvy folks with constant corrections. The latter is a common problem at the gaming table; when one sees a player making a tactical mistake, the immediate impulse is to point it out, but many players get annoyed if it happens too much. How do the other players feel about your input?
On the other hand, your DM may just be a jerk.
Here is a question I have for everyone though, which is semi-OT. Let's say we're back to the initial question, a player forgets how one power works, or that they have some bonus on, and miss with an attack. A few rounds later they suddenly remember. Do you let them retroactively apply whatever they forgot? Or does the time in between change the nature of the question? How far back would you let them "reverse time" as it were, or do you have another solution besides either allowing or disregarding the remembered bonus/ability?
Generally, if it can be retconned quickly and painlessly, I'll go ahead and do that. For instance, if a player announces, "I forgot I had a +2 to damage on my last three hits on that monster," I'll go ahead and knock off 6 more hit points. If they can make a good case that "My character wouldn't have used this power because it would have made no sense for him to do so*," I'll let them retroactively have not used it.
However, changes apply only to the present. If that 6 extra damage should have killed the monster two rounds ago, its last two rounds of actions are not retroactively erased. You don't get a new action to make up for the power you didn't use.
It can go the other way too, of course. I forget stuff now and then just like the players. However, when retconning in a monster's favor, I don't generally announce the fact. I just add 6 more points of damage to the monster's next hit.
[SIZE=-1][SIZE=-2]* Note that by "no sense," I don't mean, "tactically sub-optimal." I mean something that the character would have known was totally pointless but the player got mixed up on - e.g., using Turn Undead against creatures which are clearly alive, because the player confused Turn Undead with the
sunburst spell.[/SIZE][/SIZE]