Lots of sites a/b things, but it seems that when PMs start aggressively a/bing the core experience to the point of breaking the user journey and against just general common sense it points to a bigger product issue... i.e. "this is data driven and if we can uptick this KPI by x%", but isolated from any kind of coherent wider product vision.

It's surprisingly easy to kill a product through a series small isolated 'improvements'.

For instance, FB a/b a lot but I can't remember the last time it broke the ability to go to someone's profile entirely.