#6647: add note to two examples
diff --git a/Doc/library/warnings.rst b/Doc/library/warnings.rst
index 91ab085..dbd563e 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -173,7 +173,10 @@
While within the context manager all warnings will simply be ignored. This
allows you to use known-deprecated code without having to see the warning while
not suppressing the warning for other code that might not be aware of its use
-of deprecated code.
+of deprecated code. Note: this can only be guaranteed in a single-threaded
+application. If two or more threads use the :class:`catch_warnings` context
+manager at the same time, the behavior is undefined.
+
.. _warning-testing:
@@ -211,7 +214,9 @@
when the context was entered. This prevents tests from changing the warnings
filter in unexpected ways between tests and leading to indeterminate test
results. The :func:`showwarning` function in the module is also restored to
-its original value.
+its original value. Note: this can only be guaranteed in a single-threaded
+application. If two or more threads use the :class:`catch_warnings` context
+manager at the same time, the behavior is undefined.
When testing multiple operations that raise the same kind of warning, it
is important to test them in a manner that confirms each operation is raising