diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index d064dc1..44296f0 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -1,4 +1,4 @@
-*testing.txt*	For Vim version 8.2.  Last change: 2021 Aug 15
+*testing.txt*	For Vim version 8.2.  Last change: 2021 Dec 11
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -65,7 +65,8 @@
 		Like garbagecollect(), but executed right away.  This must
 		only be called directly to avoid any structure to exist
 		internally, and |v:testing| must have been set before calling
-		any function.
+		any function.  This will not work when called from a :def
+		function, because variables on the stack will be freed.
 
 
 test_garbagecollect_soon()			 *test_garbagecollect_soon()*