diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
index a5f8b0403f8031..324abdeb5eb5bb 100644
--- a/Doc/library/typing.rst
+++ b/Doc/library/typing.rst
@@ -1687,6 +1687,9 @@ Introspection helpers
    For a typing object of the form ``X[Y, Z, ...]`` these functions return
    ``X`` and ``(Y, Z, ...)``. If ``X`` is a generic alias for a builtin or
    :mod:`collections` class, it gets normalized to the original class.
+   If ``X`` is a :class:`Union` contained in another generic type,
+   the order of ``(Y, Z, ...)`` may be different from the order of
+   the original arguments ``[Y, Z, ...]`` due to type caching.
    For unsupported objects return ``None`` and ``()`` correspondingly.
    Examples::