diff --git a/docs/source/common_issues.rst b/docs/source/common_issues.rst
index e4239bd7a8ee..d1a13efa4f7f 100644
--- a/docs/source/common_issues.rst
+++ b/docs/source/common_issues.rst
@@ -33,6 +33,9 @@ Example:
     def foo(a):
         return '(' + a.split() + ')'  # No error!
 
+    This often surprises beginners, because mypy only checks functions that have type annotations by default.
+
+
 This gives no error even though ``a.split()`` is "obviously" a list
 (the author probably meant ``a.strip()``).  The error is reported
 once you add annotations: