The judge or jury would likely wonder:
  • How you confused the jeans you own (no tags, no security devices) with jeans that still had numerous store and manufacturer tags, and possibly also a security device;
  • How old the jeans were that you accidentally switched for the new pair

If you testify to explain what happened, assuming you have already gone through a probation violation proceeding and are convicted for the first shoplifting incident, the prosecutor will probably be able to use that incident to impeach your honesty in your testimony. (Shoplifting is theft, and thus is considered to relate to your honesty.) That would likely make it harder for the jury to accept this as an accident, even if they were instructed to consider the prior conviction only in relation to your honesty. They'll also probably wonder why somebody who can afford $270 jeans was shoplifting late last year.