Even in that sort of style, I still think this episode is a bit of an outlier. The classic example of what you describe would by the player, at time 1, saying that his/her PC puts a valuable object into his/her backpack, and then at time 2 declaring that his/her PC leaves the backpack behind.Now in other games it's up to the player to remember the important details to avoid having their pcs doing dumb things. In this kind of a game it wouldn't be a dm error.
In those circumstances, the player has made a mistake and lost the valuable object. And in the fiction, the PC has forgotten that s/he put the object in his/her backpack.
The puzzle for me about this scenario is how, at one and the same time, the NPC purchaser could inspect the armour while the PC wasn't able to see the gauntlets and ring. This is overriding the fiction by application of a player memory test.