I think there's a bug here, from what I could learn from the Buffer unit tests.  This patch should fix it:

diff -r e12a99f2e698 src/common/packet.h
--- a/src/common/packet.h       Tue Jul 17 14:34:27 2007 +0100
+++ b/src/common/packet.h       Tue Jul 17 14:43:13 2007 +0100
@@ -343,6 +343,7 @@ Packet::AddTrailer (T const &trailer)
   uint32_t size = trailer.GetSize ();
   m_buffer.AddAtEnd (size);
   Buffer::Iterator end = m_buffer.End ();
+  end.Prev (size);
   trailer.Serialize (end);
   m_metadata.AddTrailer (trailer, size);
 }