What to do when websites change and your spider doesn't know

Empty-field-rate monitoring catches selectors that return nothing. It does not catch selectors that return something wrong. The most damaging form of schema drift is the kind where a selector keeps producing values, the values are syntactically reasonable, and they are no longer the values you wanted. A price selector that quietly starts returning the financing instalment instead of the sticker price will pass every non-empty check while corrupting your data for as long as the drift goes unnotic