I've heard quite a few women, IRL and online, describe themselves as having had a "contraception failure" after having unprotected sex while using birth control apps (Natural Cycles is the one I've heard mentioned most commonly). They tend to get annoyed when they're told that they really should have used contraception - they insist that they did, the phrase "hormone-free contraceptive" is right there in the product description, etc. AIBU to be shocked that this isn't considered a violation of trading standards? It's false advertising, IMO.