Jquery load problem — for one person only

I have a page on my website that was visited 96 times. Only 1 page visit generated the javascript error “Uncaught TypeError: $ is not a function” which I understand means that the jquery library didn’t load properly. What I don’t understand is:
(a) How could this problem happen to this person when it didn’t to anyone else.
(b) What can I add to my code to detect the problem and handle it. Note that I store the useragent of each visit and the visitor in question had the same useragent as many other page visits that did not have the problem.

NOTE: The last several questions I posted on this website were rejected for various reasons, so if this happen to this question, I’d be super grateful if someone could please give me specific instructions as to what I can do to make this question accepted.