diff --git a/libs/instancelib.h b/libs/instancelib.h
index 913288b6..a36a74d9 100644
--- a/libs/instancelib.h
+++ b/libs/instancelib.h
@@ -118,12 +118,12 @@ void forEachInstance( const scene::Instantiable::Visitor& visitor ){
 }
 
 void insert( scene::Instantiable::Observer* observer, const scene::Path& path, scene::Instance* instance ){
-	ASSERT_MESSAGE( m_instances.find( key_type( observer, PathConstReference( instance->path() ) ) ) == m_instances.end(), "InstanceSet::insert - element already exists" );
-	m_instances.insert( InstanceMap::value_type( key_type( observer, PathConstReference( instance->path() ) ), instance ) );
+	const bool inserted = m_instances.insert( InstanceMap::value_type( key_type( observer, PathConstReference( instance->path() ) ), instance ) ).second;
+	ASSERT_MESSAGE( inserted, "InstanceSet::insert - element already exists" );
 }
 scene::Instance* erase( scene::Instantiable::Observer* observer, const scene::Path& path ){
-	ASSERT_MESSAGE( m_instances.find( key_type( observer, PathConstReference( path ) ) ) != m_instances.end(), "InstanceSet::erase - failed to find element" );
 	InstanceMap::iterator i = m_instances.find( key_type( observer, PathConstReference( path ) ) );
+	ASSERT_MESSAGE( i != m_instances.end(), "InstanceSet::erase - failed to find element" );
 	scene::Instance* instance = i->second;
 	m_instances.erase( i );
 	return instance;
diff --git a/libs/moduleobservers.h b/libs/moduleobservers.h
index 1ee91e27..6476bee2 100644
--- a/libs/moduleobservers.h
+++ b/libs/moduleobservers.h
@@ -35,12 +35,12 @@ public:
 	ASSERT_MESSAGE( m_observers.empty(), "ModuleObservers::~ModuleObservers: observers still attached" );
 }
 void attach( ModuleObserver& observer ){
-	ASSERT_MESSAGE( m_observers.find( &observer ) == m_observers.end(), "ModuleObservers::attach: cannot attach observer" );
-	m_observers.insert( &observer );
+	const bool inserted = m_observers.insert( &observer ).second;
+	ASSERT_MESSAGE( inserted, "ModuleObservers::attach: cannot attach observer" );
 }
 void detach( ModuleObserver& observer ){
-	ASSERT_MESSAGE( m_observers.find( &observer ) != m_observers.end(), "ModuleObservers::detach: cannot detach observer" );
-	m_observers.erase( &observer );
+	const bool erased = m_observers.erase( &observer );
+	ASSERT_MESSAGE( erased, "ModuleObservers::detach: cannot detach observer" );
 }
 void realise(){
 	for ( Observers::iterator i = m_observers.begin(); i != m_observers.end(); ++i )
diff --git a/plugins/entity/targetable.h b/plugins/entity/targetable.h
index bea09bdf..52924e94 100644
--- a/plugins/entity/targetable.h
+++ b/plugins/entity/targetable.h
@@ -424,12 +424,12 @@ typedef std::set<TargetableInstance*> TargetableInstances;
 TargetableInstances m_instances;
 public:
 void attach( TargetableInstance& instance ){
-	ASSERT_MESSAGE( m_instances.find( &instance ) == m_instances.end(), "cannot attach instance" );
-	m_instances.insert( &instance );
+	const bool inserted = m_instances.insert( &instance ).second;
+	ASSERT_MESSAGE( inserted, "cannot attach instance" );
 }
 void detach( TargetableInstance& instance ){
-	ASSERT_MESSAGE( m_instances.find( &instance ) != m_instances.end(), "cannot detach instance" );
-	m_instances.erase( &instance );
+	const bool erased = m_instances.erase( &instance );
+	ASSERT_MESSAGE( erased, "cannot detach instance" );
 }
 
 void renderSolid( Renderer& renderer, const VolumeTest& volume ) const {
diff --git a/radiant/filters.cpp b/radiant/filters.cpp
index bf34d89c..34105935 100644
--- a/radiant/filters.cpp
+++ b/radiant/filters.cpp
@@ -96,13 +96,13 @@ void addFilter( Filter& filter, int mask ){
 	g_filters.back().update();
 }
 void registerFilterable( Filterable& filterable ){
-	ASSERT_MESSAGE( g_filterables.find( &filterable ) == g_filterables.end(), "filterable already registered" );
 	filterable.updateFiltered();
-	g_filterables.insert( &filterable );
+	const bool inserted = g_filterables.insert( &filterable ).second;
+	ASSERT_MESSAGE( inserted, "filterable already registered" );
 }
 void unregisterFilterable( Filterable& filterable ){
-	ASSERT_MESSAGE( g_filterables.find( &filterable ) != g_filterables.end(), "filterable not registered" );
-	g_filterables.erase( &filterable );
+	const bool erased = g_filterables.erase( &filterable );
+	ASSERT_MESSAGE( erased, "filterable not registered" );
 }
 };
 
diff --git a/radiant/renderstate.cpp b/radiant/renderstate.cpp
index bd1fc971..d01eb433 100644
--- a/radiant/renderstate.cpp
+++ b/radiant/renderstate.cpp
@@ -1437,13 +1437,13 @@ void changed( LightCullable& cullable ){
 	( *i ).second.lightsChanged();
 }
 void attach( RendererLight& light ){
-	ASSERT_MESSAGE( m_lights.find( &light ) == m_lights.end(), "light could not be attached" );
-	m_lights.insert( &light );
+	const bool inserted = m_lights.insert( &light ).second;
+	ASSERT_MESSAGE( inserted, "light could not be attached" );
 	changed( light );
 }
 void detach( RendererLight& light ){
-	ASSERT_MESSAGE( m_lights.find( &light ) != m_lights.end(), "light could not be detached" );
-	m_lights.erase( &light );
+	const bool erased = m_lights.erase( &light );
+	ASSERT_MESSAGE( erased, "light could not be detached" );
 	changed( light );
 }
 void changed( RendererLight& light ){
@@ -1467,13 +1467,13 @@ mutable bool m_traverseRenderablesMutex;
 // renderables
 void attachRenderable( const Renderable& renderable ){
 	ASSERT_MESSAGE( !m_traverseRenderablesMutex, "attaching renderable during traversal" );
-	ASSERT_MESSAGE( m_renderables.find( &renderable ) == m_renderables.end(), "renderable could not be attached" );
-	m_renderables.insert( &renderable );
+	const bool inserted = m_renderables.insert( &renderable ).second;
+	ASSERT_MESSAGE( inserted, "renderable could not be attached" );
 }
 void detachRenderable( const Renderable& renderable ){
 	ASSERT_MESSAGE( !m_traverseRenderablesMutex, "detaching renderable during traversal" );
-	ASSERT_MESSAGE( m_renderables.find( &renderable ) != m_renderables.end(), "renderable could not be detached" );
-	m_renderables.erase( &renderable );
+	const bool erased = m_renderables.erase( &renderable );
+	ASSERT_MESSAGE( erased, "renderable could not be detached" );
 }
 void forEachRenderable( const RenderableCallback& callback ) const {
 	ASSERT_MESSAGE( !m_traverseRenderablesMutex, "for-each during traversal" );
diff --git a/radiant/undo.cpp b/radiant/undo.cpp
index ababeecf..82ba81da 100644
--- a/radiant/undo.cpp
+++ b/radiant/undo.cpp
@@ -340,12 +340,12 @@ void clear(){
 	trackersClear();
 }
 void trackerAttach( UndoTracker& tracker ){
-	ASSERT_MESSAGE( m_trackers.find( &tracker ) == m_trackers.end(), "undo tracker already attached" );
-	m_trackers.insert( &tracker );
+	const bool inserted = m_trackers.insert( &tracker ).second;
+	ASSERT_MESSAGE( inserted, "undo tracker already attached" );
 }
 void trackerDetach( UndoTracker& tracker ){
-	ASSERT_MESSAGE( m_trackers.find( &tracker ) != m_trackers.end(), "undo tracker cannot be detached" );
-	m_trackers.erase( &tracker );
+	const bool erased = m_trackers.erase( &tracker );
+	ASSERT_MESSAGE( erased, "undo tracker cannot be detached" );
 }
 void trackersClear() const {
 	for ( Trackers::const_iterator i = m_trackers.begin(); i != m_trackers.end(); ++i )