A sale price is two numbers and a line through one of them. Put the line through the wrong one and your sale becomes a price rise.
A homeware retailer I work with launched a sticky buying bar on mobile. It follows you down the product page so the button never leaves your thumb. Two days later I opened a reclaimed elm dining table on my phone. The page said £2,100, down from £2,200. The bar said £2,200, with £2,100 crossed out.
Every product on sale was doing it. 341 of 1,773.
Here is how. The bar never asked the product what it cost. It stood outside the shop window copying the price tags, and took the first number it saw as the real one. The theme prints the old price first, then strikes it through. The bar copied the numbers and missed the pen.
The launch checks passed, because they checked that a price appeared. Nobody checked the order.
The fix was to stop reading the window. The bar now takes each price straight from the product’s own data, for whichever option the customer has picked, the same record the checkout charges from.
The same pass turned up a quieter cousin. The page’s plus and minus buttons changed the quantity without telling the bar. The form said two. The bar said one. One tap and a customer would have added two of something while the bar promised a single.
Anything that copies the page is not showing you the product. It is a very confident tourist reading the menu through the glass.