jQuery calls error callback on successful ajax request (status 200)

Sometimes though the jQuery ajax code block is right, it goes to the error condition block. we found no clues on this kind of strange behavior. Why this happened?
Actually, the request successfully reached the destination server, but when it returns the client expects one kind of response, but the server returns in different format. this produces the error.
It was expecting a Json response, but it was getting empty result, that's why it was showing on the error callback though it was a successful call.
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