WordPress site. SEO plugin auto-updated and changed our meta descriptions and title tags to defaults without notifying us. Every page on the site suddenly had generic titles.
Rankings dropped from page one to page three for our primary keywords over about two weeks. Organic traffic fell 55%.
Didn't notice for 19 days because nobody was monitoring the actual on-page SEO after the plugin update. We check rankings weekly but we weren't checking whether the content behind those rankings had silently changed.
Fixed the titles and descriptions. Rankings recovered over about six weeks. Lost roughly $4,200 in revenue during the recovery period.
Auto-updates on SEO plugins are now disabled. Every update is manual, reviewed, and tested. And we run a monthly audit that checks whether our actual page titles match what we intended them to be.
The thing that broke our SEO wasn't a competitor or an algorithm change. It was a plugin doing exactly what we told it to do.