Anything with Drag & Drop functionality. When users press inside the draggable to try and move it, sometimes text will select instead and need to be cleared before dragging (particularly annoying on mobile devices).

Also video controls that use the UNICODE characters for e.g. play/pause/stop/etc instead of graphic icons (so it scales better at higher DPI). You don't want people to be able to select the text inside the button when pushing the button.

There's actually tons of great usage of user-select: none. This is definitely not one of them though.