What stage is your application?
What is your CRS score?
Was the job experience in the last 10 years?
Is the job that you missed out, with the same NOC as your other work experience that you are claiming points for?
Will you gain any additional points by adding that particular job experience?
I think the general advice is that you should update your application with all of the relevant details, even if you may have accidentally omitted something before, but depending on your answers to the above, it may not make a huge difference to your application.