I imagine if an ETF accidentally exceeded 15% then the board would go talk to the fund managers and maybe get them to sign some documents. This isn't a computer program that will automatically trigger armageddon. It is just some new rules that mean if you turn up at a board meeting saying 'haha, I just bought your company and you are all fired', the board gets to say 'nope, we just issued a bazillion shares and gave them to all the existing holders except you, and you now own just 14.9%, sorry not sorry for your loss'.