A boiled down example from Anthony's MIME torture tests. This one has a

separating boundary for an outer part inside an inner part.

Lib/email/test/data/msg_38.txt

+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary="----- =_aaaaaaaaaa0"
+
+------- =_aaaaaaaaaa0
+Content-Type: multipart/mixed; boundary="----- =_aaaaaaaaaa1"
+Content-ID: <20592.1022586929.1@example.com>
+
+------- =_aaaaaaaaaa1
+Content-Type: multipart/alternative; boundary="----- =_aaaaaaaaaa2"
+Content-ID: <20592.1022586929.2@example.com>
+
+------- =_aaaaaaaaaa2
+Content-Type: text/plain
+Content-ID: <20592.1022586929.3@example.com>
+Content-Description: very tricky
+Content-Transfer-Encoding: 7bit
+
+
+Unlike the test test_nested-multiples-with-internal-boundary, this
+piece of text not only contains the outer boundary tags 
+------- =_aaaaaaaaaa1 
+and 
+------- =_aaaaaaaaaa0 
+but puts them at the start of a line! And, to be even nastier, it
+even includes a couple of end tags, such as this one:
+
+------- =_aaaaaaaaaa1--
+
+and this one, which is from a multipart we haven't even seen yet!
+
+------- =_aaaaaaaaaa4--
+
+This will, I'm sure, cause much breakage of MIME parsers. But, as 
+far as I can tell, it's perfectly legal. I have not yet ever seen
+a case of this in the wild, but I've seen *similar* things.
+
+
+------- =_aaaaaaaaaa2
+Content-Type: application/octet-stream
+Content-ID: <20592.1022586929.4@example.com>
+Content-Description: patch2
+Content-Transfer-Encoding: base64
+
+XXX
+
+------- =_aaaaaaaaaa2--
+
+------- =_aaaaaaaaaa1
+Content-Type: multipart/alternative; boundary="----- =_aaaaaaaaaa3"
+Content-ID: <20592.1022586929.6@example.com>
+
+------- =_aaaaaaaaaa3
+Content-Type: application/octet-stream
+Content-ID: <20592.1022586929.7@example.com>
+Content-Description: patch3
+Content-Transfer-Encoding: base64
+
+XXX
+
+------- =_aaaaaaaaaa3
+Content-Type: application/octet-stream
+Content-ID: <20592.1022586929.8@example.com>
+Content-Description: patch4
+Content-Transfer-Encoding: base64
+
+XXX
+
+------- =_aaaaaaaaaa3--
+
+------- =_aaaaaaaaaa1
+Content-Type: multipart/alternative; boundary="----- =_aaaaaaaaaa4"
+Content-ID: <20592.1022586929.10@example.com>
+
+------- =_aaaaaaaaaa4
+Content-Type: application/octet-stream
+Content-ID: <20592.1022586929.11@example.com>
+Content-Description: patch5
+Content-Transfer-Encoding: base64
+
+XXX
+
+------- =_aaaaaaaaaa4
+Content-Type: application/octet-stream
+Content-ID: <20592.1022586929.12@example.com>
+Content-Description: patch6
+Content-Transfer-Encoding: base64
+
+XXX
+
+------- =_aaaaaaaaaa4--
+
+------- =_aaaaaaaaaa1--
+
+------- =_aaaaaaaaaa0
+Content-Type: text/plain; charset="us-ascii"
+Content-ID: <20592.1022586929.15@example.com>
+
+--
+It's never too late to have a happy childhood.
+
+------- =_aaaaaaaaaa0--