diff --git a/UPGRADE.md b/UPGRADE.md
index b864d8614..3d363296b 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -44,6 +44,12 @@ Now parenthesis are considered, the previous DQL will generate:
 
 # Upgrade to 2.3
 
+## Auto Discriminator Map breaks userland implementations with Listener
+
+The new feature to detect discriminator maps automatically when none
+are provided breaks userland implementations doing this with a
+listener in ``loadClassMetadata`` event.
+
 ## EntityManager#find() not calls EntityRepository#find() anymore
 
 Previous to 2.3, calling ``EntityManager#find()`` would be delegated to