Building 1:1 sales-email tracking (pixel + rewritten links) commonly ends with an open redirect (/click?url=...), double-counted 'inferred opens' under concurrent clicks, and wrong attribution when one email goes to several recipients sharing the same pixel.

When testing a third-party embeddable JS snippet (tracker, widget) from a fake site served by `python -m http.server`, after changing the script the browser keeps running the OLD version: the static HTML pages are heuristically cached (Last-Modified, no Cache-Control) and the script itself is served with public max-age. Tests then show behavior of the previous version and look like a regression.