diff --git a/crates/bevy_app/src/app.rs b/crates/bevy_app/src/app.rs
index f79bc6d52a..07a29800f9 100644
--- a/crates/bevy_app/src/app.rs
+++ b/crates/bevy_app/src/app.rs
@@ -990,6 +990,36 @@ impl App {
     }
 
     /// Spawns an [`Observer`] entity, which will watch for and respond to the given event.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use bevy_app::prelude::*;
+    /// # use bevy_ecs::prelude::*;
+    /// # use bevy_utils::default;
+    /// #
+    /// # let mut app = App::new();
+    /// #
+    /// # #[derive(Event)]
+    /// # struct Party {
+    /// #   friends_allowed: bool,
+    /// # };
+    /// #
+    /// # #[derive(Event)]
+    /// # struct Invite;
+    /// #
+    /// # #[derive(Component)]
+    /// # struct Friend;
+    /// #
+    /// // An observer system can be any system where the first parameter is a trigger
+    /// app.observe(|trigger: Trigger<Party>, friends: Query<Entity, With<Friend>>, mut commands: Commands| {
+    ///     if trigger.event().friends_allowed {
+    ///         for friend in friends.iter() {
+    ///             commands.trigger_targets(Invite, friend);
+    ///         }
+    ///     }
+    /// });
+    /// ```
     pub fn observe<E: Event, B: Bundle, M>(
         &mut self,
         observer: impl IntoObserverSystem<E, B, M>,
diff --git a/crates/bevy_ecs/src/system/observer_system.rs b/crates/bevy_ecs/src/system/observer_system.rs
index c5a04f25dd..28ea902b1a 100644
--- a/crates/bevy_ecs/src/system/observer_system.rs
+++ b/crates/bevy_ecs/src/system/observer_system.rs
@@ -25,6 +25,11 @@ impl<
 }
 
 /// Implemented for systems that convert into [`ObserverSystem`].
+#[diagnostic::on_unimplemented(
+    message = "`{Self}` cannot become an `ObserverSystem`",
+    label = "the trait `IntoObserverSystem` is not implemented",
+    note = "for function `ObserverSystem`s, ensure the first argument is a `Trigger<T>` and any subsequent ones are `SystemParam`"
+)]
 pub trait IntoObserverSystem<E: 'static, B: Bundle, M, Out = ()>: Send + 'static {
     /// The type of [`System`] that this instance converts into.
     type System: ObserverSystem<E, B, Out>;