In the scenario described here, it was a big enough battle that one Confine probably would not have “saved the day.” However, the problem was that I had had four Confines memorized plus several lower-level binding spells, and due to the NPC's mistake I thought that the kobolds we were fighting (they all looked the same) actually could rip Confines, and because of that didn't use any of the binding spells. So the mistake didn't just prevent the one Confine effect, it prevented lots of other effects. (We did end up winning the fight, but it was close - the kobolds were throwing lots of gasses and there were times when almost half the group was incapacitated.)
There are also other ways this could have turned out. If I had used the Confine on another kobold and it couldn't have ripped free, I might have ended up thinking there was something special about the first kobold and altered battle tactics based on that. And after the battle I was considering using downtime actions and other in-game resources to try to investigate how kobolds gained the ability to rip Confines, based on something that wasn't supposed to have happened.
Of course this didn't happen because the Marshal checked the card after the fight and told me, but it seems like in order to avoid being led down the wrong path due to an NPC mistake, it would be necessary to check with Plot after every battle where something unexpected happens. For instance, let's say I'm fighting flame elementals and that rather than being healed by flame, they're being healed by lightning. How can I tell whether this is because there's something unusual IG about these flame elementals (in which case I might try to investigate further IG) or because the NPC got the elements mixed up (in which case I should pretend that I didn't observe that)?
What can players do to avoid these types of situations, and what can Plot do to make this easier for players?
One suggestion might be in these situations is for example, in the case of the kobold who could rip Confines, rather than Plot retconning it as “you never cast the Confine, take your spell back,” retcon it instead as “well, actually, it turns out that these kobolds really are special and can rip Confines, let's put out a hook to go on a cool new adventure to find out how they got that power!”
There are also other ways this could have turned out. If I had used the Confine on another kobold and it couldn't have ripped free, I might have ended up thinking there was something special about the first kobold and altered battle tactics based on that. And after the battle I was considering using downtime actions and other in-game resources to try to investigate how kobolds gained the ability to rip Confines, based on something that wasn't supposed to have happened.
Of course this didn't happen because the Marshal checked the card after the fight and told me, but it seems like in order to avoid being led down the wrong path due to an NPC mistake, it would be necessary to check with Plot after every battle where something unexpected happens. For instance, let's say I'm fighting flame elementals and that rather than being healed by flame, they're being healed by lightning. How can I tell whether this is because there's something unusual IG about these flame elementals (in which case I might try to investigate further IG) or because the NPC got the elements mixed up (in which case I should pretend that I didn't observe that)?
What can players do to avoid these types of situations, and what can Plot do to make this easier for players?
One suggestion might be in these situations is for example, in the case of the kobold who could rip Confines, rather than Plot retconning it as “you never cast the Confine, take your spell back,” retcon it instead as “well, actually, it turns out that these kobolds really are special and can rip Confines, let's put out a hook to go on a cool new adventure to find out how they got that power!”