It's always fun when the content layout naturally fits into the viewport in such a way to make it look like there's nothing down below.
Also fun when websites that are waging their holy war against the bars don't anticipate that I'm zoomed in a bit. Having to delve into devtools to disable "overflow: hidden" is a delight.
Whats worse is when a website has a footer but they have a never-ending scroll div between the header and footer so as you approach the footer more content loads pushing the footer even further down.
Just this year I was trying to explain to mom how to do something with an iPhone.
There’s about a million different secret handshakes and gestures in iOS UI, but I consider the most egregious UI failure to be the “Share” button, which manages to pop up a modal in such a way that it often disguises both that it can be be scrolled vertically AND that it can be scrolled horizontally.
0. The existence of a scrollbar tells you there's something to be scrolled.
Flat UIs are pretty good at accidentally hiding the fact that not all of the content fits on screen on in whatever borderless frame.