Fix note #1 of https://github.com/mrdoob/three.js/pull/5805#issuecomment-75610227

src/renderers/shaders/ShaderChunk/lightmap_fragment.glsl

@@ -1,5 +1,5 @@
 #ifdef USE_LIGHTMAP
 
-	outgoingLight += diffuseColor.xyz * texture2D( lightMap, vUv2 ).xyz;
+	outgoingLight *= diffuseColor.xyz * texture2D( lightMap, vUv2 ).xyz;
 
 #endif