diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index c223e54..14784fa 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1646,6 +1646,10 @@
 
    Fork a child process.  Return ``0`` in the child and the child's process id in the
    parent.  If an error occurs :exc:`OSError` is raised.
+
+   Note that some platforms including FreeBSD <= 6.3, Cygwin and OS/2 EMX have
+   known issues when using fork() from a thread.
+
    Availability: Unix.