I don't know about most painful, but my dad bought a phone many months ago and last week, he wanted to know how to turn on the flashlight on it. I was ready to edit the notification dropdown or give a five step explainer or whatever.
Turns out, nope, you just pull down the notification bar and there's a pretty obvious flashlight button right there. The problem is, you see, he did not know you could drag down the notification bar. There were dozens of notifications there.
I really cannot blame him either. I don't know what UX designer came up with just putting a bar at the top and expecting users to know that you can drag on it. But yeah, still, ouch.