diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index 1ea16939603f9..55258df43e09b 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -171,6 +171,10 @@ class providing the base-class of operations.
         The resulting dtype will reflect the return value of the passed ``func``,
         see the examples below.
 
+    Functions that mutate the passed object can produce unexpected
+    behavior or errors and are not supported. See :ref:`gotchas.udf-mutation`
+    for more details.
+
     Examples
     --------
     {examples}