e.g. in Gnome when I open a media file with VLC the focus remains on Files app even if the video opened fullscreen over the top. So when I press space to pause I instead open a second preview of the same file in the background.
Or I use the Gnome media player and it turns sub-tittles back on every time I use it, despite me previously turning them off.
Or the sleep system doesn't suspend to disk after an hour of RAM usage so that my battery is flat when I return latter.
Little stuff like that is an unnecessary pain, just for a lack of polish. I persevere with Linux because of all the little things it does better but it's a hard sell to friends and family.