diff --git a/libs/pavexc/src/compiler/analyses/call_graph/core_graph.rs b/libs/pavexc/src/compiler/analyses/call_graph/core_graph.rs
index 9ab215ec8..f482df367 100644
--- a/libs/pavexc/src/compiler/analyses/call_graph/core_graph.rs
+++ b/libs/pavexc/src/compiler/analyses/call_graph/core_graph.rs
@@ -251,7 +251,6 @@ where
         let graph_size_before_transformations = indexes.len();
 
         // For each node, we add the respective transformers, if they have been registered.
-        let indexes = call_graph.node_indices().collect::<Vec<_>>();
         for node_index in indexes {
             if transformed_node_indexes.contains(&node_index) {
                 continue;
@@ -336,7 +335,7 @@ where
                     };
 
                     // There are two topologies for error handlers:
-                    // - Directly downstream of an error matcher, which in turn has `pavex::Error::new` as a child
+                    // - Directly downstream of the `Err` branch of match, which in turn has `pavex::Error::new` as a child
                     // - Directly downstream of a `pavex::Error::new` invocations
                     // We want to find the `pavex::Error::new` node index for this error.
                     let pavex_error_new_node_index = match call_graph
@@ -494,11 +493,45 @@ where
         assert_eq!(downstream_sources.len(), 1);
         downstream_sources[0]
     };
-    Ok(CallGraph {
+    enforce_invariants(
+        &call_graph,
+        error_observer_ids.len(),
+        component_db,
+        computation_db,
+    );
+
+    let call_graph = CallGraph {
         call_graph,
         root_node_index: new_root_index,
         root_scope_id,
-    })
+    };
+    Ok(call_graph)
+}
+
+fn enforce_invariants(
+    call_graph: &RawCallGraph,
+    n_unique_error_observers: usize,
+    component_db: &ComponentDb,
+    computation_db: &ComputationDb,
+) {
+    let mut n_error_observers = 0;
+    let mut n_errors = 0;
+    for node in call_graph.node_weights() {
+        if let CallGraphNode::MatchBranching { .. } = node {
+            n_errors += 1;
+            continue;
+        }
+        if let Some(HydratedComponent::ErrorObserver(_)) =
+            node.as_hydrated_component(component_db, computation_db)
+        {
+            n_error_observers += 1;
+        };
+    }
+    let expected_error_observers = n_errors * n_unique_error_observers;
+    assert_eq!(
+        expected_error_observers, n_error_observers,
+        "There should be {expected_error_observers} error observers in the graph, but we found {n_error_observers}."
+    )
 }
 
 impl CallGraph {
diff --git a/libs/pavexc/src/compiler/analyses/components/db/mod.rs b/libs/pavexc/src/compiler/analyses/components/db/mod.rs
index 3195ba1b1..f7396ad24 100644
--- a/libs/pavexc/src/compiler/analyses/components/db/mod.rs
+++ b/libs/pavexc/src/compiler/analyses/components/db/mod.rs
@@ -197,57 +197,57 @@ impl ComponentDb {
                 krate_collection,
                 diagnostics,
             );
-
-            // This **must** be invoked after `process_request_handlers` because it relies on
-            // all request handlers being registered to determine which scopes have error observers.
-            self_.process_error_observers(
-                &pavex_error_ref,
+            self_.process_wrapping_middlewares(
+                &mut needs_error_handler,
                 computation_db,
                 package_graph,
                 krate_collection,
                 diagnostics,
             );
-
-            // We process the backlog of matchers that were not registered during the initial
-            // registration phase for request handlers.
-            self_.register_all_matchers(computation_db);
-            // From this point onwards, all fallible components will automatically get matchers registered.
-            // All error matchers will be automatically paired with a conversion into `pavex::error::Error` if needed,
-            // based on the scope they belong to.
-            self_.autoregister_matchers = true;
-
-            self_.process_constructors(
+            self_.process_pre_processing_middlewares(
                 &mut needs_error_handler,
                 computation_db,
-                framework_item_db,
                 package_graph,
                 krate_collection,
                 diagnostics,
             );
-
-            self_.process_wrapping_middlewares(
+            self_.process_post_processing_middlewares(
                 &mut needs_error_handler,
                 computation_db,
                 package_graph,
                 krate_collection,
                 diagnostics,
             );
-            self_.process_pre_processing_middlewares(
-                &mut needs_error_handler,
+            self_.compute_request2middleware_chain(pavex_noop_wrap_id, computation_db);
+
+            // This **must** be invoked after request handlers and middlewares have been
+            // processed, since it needs to determine which scopes have error observers
+            // attached to them.
+            self_.process_error_observers(
+                &pavex_error_ref,
                 computation_db,
                 package_graph,
                 krate_collection,
                 diagnostics,
             );
-            self_.process_post_processing_middlewares(
+
+            // We process the backlog of matchers that were not registered during the initial
+            // registration phase for request handlers.
+            self_.register_all_matchers(computation_db);
+            // From this point onwards, all fallible components will automatically get matchers registered.
+            // All error matchers will be automatically paired with a conversion into `pavex::error::Error` if needed,
+            // based on the scope they belong to.
+            self_.autoregister_matchers = true;
+
+            self_.process_constructors(
                 &mut needs_error_handler,
                 computation_db,
+                framework_item_db,
                 package_graph,
                 krate_collection,
                 diagnostics,
             );
 
-            self_.compute_request2middleware_chain(pavex_noop_wrap_id, computation_db);
             self_.process_error_handlers(
                 &mut needs_error_handler,
                 computation_db,
@@ -850,6 +850,11 @@ impl ComponentDb {
                 continue;
             }
             v.push(self.scope_id(component_id));
+            if let Some(middleware_ids) = self.handler_id2middleware_ids.get(&component_id) {
+                for middleware_id in middleware_ids {
+                    v.push(self.scope_id(*middleware_id));
+                }
+            }
         }
         self.scope_ids_with_observers = v;
     }