The Firebird
Commoner
For me this approaches the line, but I don't think it crosses it. I'd prefer for the GM to have many of these details fixed. For example, if they had written "the firefly festival occurs at this time and involves this sort of activity". Then the player could say "maybe our characters fought sometime we were in town?" and the GM would say "how about the firefly festival...this involves...".Digging out a log, in this case, it was the NPC's sister, another noble lady, an orchard in the NPC's lands and a firefly festival in a village on the NPC's land. The grievance (as we-as-real-people discovered together) was around courtly woo-ing, accusations of unfaithfulness, wounded honour and so on, all very chaste and Arthurian. In no way did my PC have a hand in the creation of the orchard or the festival, but they would have been familiar with them due to visiting the location before. Now, all this could have been created before hand, it just happened that it hadn't been.
That seems better to me than the player declaring the festival's existence and the GM modifying the world to accommodate it.
But it wouldn't be a red flag and I'd be ok playing in a game with that level of player control.