diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -86,6 +86,10 @@
    defaults to 0.  *offset* must be a multiple of the PAGESIZE or
    ALLOCATIONGRANULARITY.
 
+   *Note:* on Mac OS X there is a known memory-mapping error for large
+   files (which extend the 4 GB limit).  You should create :class:`mmap`
+   objects only on files which have been opened recently.
+
    This example shows a simple way of using :class:`mmap`::
 
       import mmap