diff --git a/pubspec.yaml b/pubspec.yaml
index 80c7e7f..97d6c6b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,4 +1,12 @@
 name: source_maps
+
+# Note! This version number is referenced directly in the pub source code in
+# lib/src/barback.dart. Pub implicitly places a version constraint on
+# source_maps to ensure users only select a version of source_maps that works
+# with their current version of pub.
+#
+# When the minor version is upgraded, you *must* update that version constraint
+# in pub to stay in sync with this.
 version: 0.9.2+2
 author: Dart Team <misc@dartlang.org>
 description: Library to programmatically manipulate source map files.