Raise a RuntimeError when tee iterator is consumed from different threads (GH-15567)
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index a3f403a..8e7899e 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -643,7 +643,8 @@
Once :func:`tee` has made a split, the original *iterable* should not be
used anywhere else; otherwise, the *iterable* could get advanced without
- the tee objects being informed.
+ the tee objects being informed. the :func:`tee` iterator can not be consumed
+ from different threads, even if an underlying iterator is thread-safe.
This itertool may require significant auxiliary storage (depending on how
much temporary data needs to be stored). In general, if one iterator uses