twisted.web.http.PotentialDataLoss(Exception) class documentation

PotentialDataLoss may be raised by a transfer encoding decoder's noMoreData method to indicate that it cannot be determined if the entire response body has been delivered. This only occurs when making requests to HTTP servers which do not set Content-Length or a Transfer-Encoding in the response because in this case the end of the response is indicated by the connection being closed, an event which may also be due to a transient network problem or other error.