Minor issue that possibly is user error but is catching me out quite a bit:
If I hover over the notification bell to get the popup, the popup frequently drops down when I try to move to it. (Edit: this is on desktop/Firefox)
One cause is that if you hover over the notification bell, the popup menu is basically right-aligned with the edge of the bell icon. In other words, if I were to drag down and right, I would likely miss the popup menu. So my tendency is to drag left of the bell icon while moving down - if I go too far left it pops down.
I thought that was the cause (and so it was kind of an ergonomics issue causing user error), but there are definitely times when I drag straight down and get the same pop-down effect. It's not totally repeatable, but it's around a 50/50 guess whether it will pop down.