diff --git a/framework/UPGRADE.md b/framework/UPGRADE.md
index aa550ce658..c308ccfcc0 100644
--- a/framework/UPGRADE.md
+++ b/framework/UPGRADE.md
@@ -284,7 +284,10 @@ Upgrade from Yii 2.0.13
   - If you are using XCache or Zend data cache, those are going away in 2.1 so you might want to start looking for an alternative.
 
 * In case you aren't using CSRF cookies (REST APIs etc.) you should turn them off explicitly by setting
-  `\yii\web\Request::$enableCsrfCookie` to `false` in your config file. 
+  `\yii\web\Request::$enableCsrfCookie` to `false` in your config file.
+  
+* Previously headers sent after content output was started were silently ignored. This behavior was changed to throwing
+  `\yii\web\HeadersAlreadySentException`.
 
 Upgrade from Yii 2.0.12
 -----------------------