A Green API Is Not a Working Page

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Five production bugs that passed every API check while the page was broken — including an HTML escape that killed the click handler on a third of the cards.
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Every endpoint returned 200. Every latency measurement was green. Meanwhile a third of the product cards had a dead click handler, a call-to-action had been inert long enough for a shopper to click it five times in 26 seconds on a session recording, and product images sat fully loaded inside cards that were one pixel tall. None of it was visible from the server, because a handler that never compiles leaves no trace there. Here is each bug, the measurement that actually found it, and the assertion I now write instead.