diff --git a/reference/forms/types/entity.rst b/reference/forms/types/entity.rst
index af7992c5c8c..1fa0b9e8774 100644
--- a/reference/forms/types/entity.rst
+++ b/reference/forms/types/entity.rst
@@ -191,6 +191,13 @@ The value of this option can either be a ``QueryBuilder`` object, a Closure or
 passed the ``EntityRepository`` of the entity as the only argument and should
 return a ``QueryBuilder``.
 
+.. caution::
+
+    The entity used in the ``FROM`` clause of `query_builder`_ option
+    will always be validated against the class which you have specified with the form's `class`_ option.
+    If you return another entity instead of the one used in your ``FROM`` clause
+    (for instance if you return an entity from a joined table) it will break validation.
+    
 Overridden Options
 ------------------