An unknown error 100 occurred

It means that it is a provisional response wherein just indicating that optional header and line of status is working and it is less frequently shown to browser
by server.
and if this error or request comes as server tell the user agent that some of the part has been received and user must continue what is left to complete the process and
if the user has already completed the 100 continue request then he must be leaving it as it is as user already clicked on the continue button.
and when user has received the user request from 100 response .
second reason is that if the request is large one and cannot be completed by server by one request, user must continue to process it wholly.