I saw that one item on a website was not displaying comments when it should have, and decided to see why. It turns out someone posted a comment ending with at :

I saw that one item on a website was not displaying comments when it should have, and decided to see why. It turns out someone posted a comment ending with at :\ smiley, and the comments were passed in JSON with backslashes not being escaped. So the \ escaped the ending " causing it to fail. I quickly used the unescaped backslashes to insert < and got an XSS.