What's an example of a "bad" letter of recommendation?

<p>Most of my teachers tend to write single paragraphs when writing recommendations. Not sure if that's good or bad.
Also, what sort of proof are we supposed to have that the teachers actually wrote the letters?
Should I have them sign it?</p>

<p>I think [this</a> link](<a href=“http://mitadmissions.org/apply/prepare/writingrecs]this”>How to write good letters of recommendation | MIT Admissions) may answer some questions.</p>