Panic when a despawning Entity's observer spawns an Entity

[Yttrmin]

Bevy version

0.14.0

What you did

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(MinimalPlugins)
        .add_systems(Startup, startup)
        .add_systems(Update, update)
        .run();
}

#[derive(Component)]
struct Marker;

fn startup(mut commands: Commands) {
    commands.spawn(Marker).observe(|_: Trigger<OnRemove, Marker>, mut commands: Commands| {
        commands.spawn_empty();
    });
}

fn update(
    query: Query<Entity, With<Marker>>,
    mut commands: Commands
) {
    for entity in &query {
        commands.entity(entity).despawn();
    }
}

What went wrong

What were you expecting?

1. Entity with Marker component gets queued for despawning.
2. Its observer runs and queues the spawning of a new entity.
3. End result is the entity with Marker being despawned, and a new empty entity being spawned.
4. App continues to run.

What actually happened?

thread 'main' panicked at F:\packages\cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\entity\mod.rs:582:9:
flush() needs to be called before this operation is legal
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic when applying buffers for system `flush_panic_repro::update`!
Encountered a panic in system `bevy_app::main_schedule::Main::run_main`!
error: process didn't exit successfully: `target\debug\flush_panic_repro.exe` (exit code: 101)

Additional information

#14300 runs into the same panic, but involves component hooks, MapEntities, and DynamicScene. I don't know enough about Bevy's internals to know if these are ultimately the same bug, or just coincidentally panic in the same spot.

---

If just the Marker component is removed without despawning its entity, the observer still runs (as expected) but there is no panic:

fn update(
    query: Query<Entity, With<Marker>>,
    mut commands: Commands
) {
    for entity in &query {
        commands.entity(entity).remove::<Marker>(); // Does NOT panic.
    }
}
// Rest of code is unchanged.

---

Here's what seems like the most relevant part of the panic backtrace:

2: bevy_ecs::entity::Entities::verify_flushed
   at F:\packages\cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\entity\mod.rs:582
3: bevy_ecs::entity::Entities::free
   at F:\packages\cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\entity\mod.rs:680
4: bevy_ecs::world::entity_ref::EntityWorldMut::despawn
   at F:\packages\cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\world\entity_ref.rs:1235
5: bevy_ecs::world::World::despawn
   at F:\packages\cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\world\mod.rs:1105
6: bevy_ecs::system::commands::despawn
   at F:\packages\cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_ecs-0.14.0\src\system\commands\mod.rs:1241