What could be the outcome of a poorly written subject line?

The outcome of a poorly written subject line is that it won't be able to convey the message of what the writer would like to say to the reader or to the one that he or she had sent it to, It is because a subject line is the idea or tries to pertain and point out what is the message about. If it is written poorly, then it would only confuse the reader and in the same time there is no understanding and connection of the subject line to the message.