The following code exists in the HM decoder in TDecTop::xGetNewPicBuffer:

  if ( !bBufferIsAvailable )
  {
    //There is no room for this picture, either because of faulty encoder or dropped NAL. Extend the buffer.
    m_iMaxRefPicNum++;
    rpcPic = new TComPic();
    m_cListPic.pushBack( rpcPic );
  }

It basically allocates memory more than the size indicated by the DPB, which should not be permitted. It is particularly significant in testing bitstream conformance, to ensure that a bitstream does not accidentally exceed the signalled DPB size but HM decoder doesn't catch it.