Index: doc/library/warnings.rst
===================================================================
--- doc/library/warnings.rst	(revision 76294)
+++ doc/library/warnings.rst	(working copy)
@@ -178,7 +178,9 @@
 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:
@@ -216,7 +218,8 @@
 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: If two or more threads alter the warnings filter 
+at the same time, its final state 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