So recently I was fixing some jquery spaghetti (the bosses didn't allow refactoring, because 'We already have so much code, why waste it?' or something; fortunately they changed their mind since). I decided to display a native alert when some rare error occured (instead of failing silently). Shit, it doesn't work, I must have broken something somewhere... Then I found this gem, in a file included from somewhere 7 directories away:
window.alert = function() {};
This line appeared in a commit with an optimistic name "last fixes", in January 2017. The project is still under active development, with a completely new team and after total rewrite of about 60% of functionality.