My most hated one is not allowing me to right-click to open a context menu, or even worse, making right-click act the same as left-click, redirecting me to some page I never intended to go to.
One thing is really dislike about modern UI design is where you see a headline that seems kinda interesting but you don't click on it right away. Come back a bit later and it's just gone. There was some A/B testing on the headline and now it's completely different, probably now all clickbaity so I won't be inclined to check it out.
Crazy to add complexity like this.