Exploiting enemy A.I. and Game Mechanics
While the NWN engine is indeed quite capable of creating interesting worlds to explore and providing a decent translation of the D&D rules into a digital medium, it definitely also has its flaws and drawbacks. Most of these come in the form of engine limitations and gaps in the artificial intelligence itself, and this can often be exploited by players in a way that would be inconsistent within an RP environment.
A common example of this would be exploiting gaps and quirks in monster A.I. in order to maximize the amount of experience gained from enemies and thus "farming" exp. For instance, a map could feature a large assortment of enemies that will tend to swarm the players because of their A.I. scripts. Using powerful area of effect spells such as Fireball or Hammer of the Gods after herding all of them together into one concentrated mass would be considered exploiting the enemy A.I. Players should refrain from doing this, and repetition of it in a single area would most definitely be construed as an exploit.
Another example is the use of "chokepoints" to effectively trap monsters and render them helpless. There is a fine line between the use of tactical chokepoints and actually getting monsters stuck, and it should be quite apparent to the players which is which. Using tactical chokepoints is acceptable and often a good idea. Getting monsters stuck on terrain features is not. Getting creatures stuck on terrain to kill them, repeatedly would be considered an exploit.
Invisibility, sanctuary, greater sanctuary and similar spells should also be used appropriately, and never used to exploit creature AI or bypass spawns when it does not make sense to do so. Players should refrain from using these spells to bypass a number of spawns in an area only to engage the last spawn guarding CNR, etc., especially when the creatures in those spawns would have other means of detecting your character (e.g. listen checks, visible footprints, and so on). Players should use common sense, refrain from doing anything that does not "feel right", and ask the GM Team for clarification if they are unsure whether a use of these spells is okay or not.
This is by no means an exhaustive list of all possible exploits of the game engine as there are always new ones cropping up from time to time ranging from monster A.I. to such things as item duplication, etc. As before, it is ultimately up to the players and their own common sense. If it feels like an exploit (i.e. too cheap and easy) then it most likely is.
