We checked 13 yoga mat listings in one day. Three pointed at the wrong product.

The first time we asked Amazon's API for prices on our catalogue, three of thirteen listings had stopped being the mat we recorded them as.

One had been a Manduka PROlite. By the time we checked, the same product code served a listing titled "Manduka PRO Yoga Mat – Multipurpose Exercise Mat". Different mat, different thickness, different price. Had we trusted the number, this site would have published the PRO's price on the PROlite's page and never known.

Another was a listing we had filed as the 68-inch PROlite. The title said 71 inches. Our check compares the length in the title against the length we expect, and it refused the price. That one was our filing error rather than Amazon's, but the effect is identical: a number attached to the wrong thing.

Why this happens

Amazon product codes are not permanent labels for a physical object. Sellers merge listings, brands consolidate colour variants, and a code that pointed at one product in March can serve another by August. Nothing announces the change. The listing keeps its reviews, its ranking and its URL.

For a site that publishes prices, that is not a small risk. An unverified price is not slightly wrong; it is the right number for the wrong product, presented with total confidence.

What we do about it

Every price we receive is checked against the model it is supposed to belong to before we believe it. The title has to carry the brand and the product line. The thickness has to match within ten percent, because "6 mm" is routinely sold as 5.5 mm. The length has to match if the title states one. Anything that mentions a bag, a strap or a towel is thrown out, because accessories carry the same brand name.

When a listing fails, we keep the previous price and flag the product rather than publish the new figure. When it fails because the listing now shows a different product, we drop it immediately — that is proof, not noise.

The tedious part is that this has to run every day, forever. The useful part is that it means a price on this site is attached to the mat it claims to describe.