I once encountered this in the real world as a data analyst a long time ago. I was working at an e-commerce company, called The Hut Group, and the whole year our marketing team had been saying our marketing cost of goods sold (the percentage of our revenue we needed to spend on marketing) had been declining across every product category. But at year end, the execs were shocked to realize that our cost of goods sold had almost doubled, from 10% to nearly 20%.

The finance team had asked me to double check the marketing team's numbers, to see if there'd been some funny math in the reporting. But the marketing team were totally right, marketing spend across the three main categories - games, beauty, and nutrition had all fallen (~15% to ~10%, ~30% to ~25%, and ~50% to ~30% respectively). However, the mix of these product categories had shifted massively, with nutrition growing from roughly 10% of our total sales to now nearly 50%.

In net that meant that whilst the marketing team had gotten more cost-efficient at selling every individual product category, the growth in the nutrition industry had vastly outstripped the growth in all other categories, and since that was the highest individual category, the aggregate marketing costs % had gone up, even though the team had improved every category. I then had the fun job of explaining the Yule Simpson paradox to a bunch of accountants.