I found that an interesting example and I struggle with classification (which is quite important for our metrics): There is an issue with a hardware button, when it is kept pressed, it toggles the ON/OFF conditions (as if holding a power button caused continual turning on/turning off of a device). The action should happen only once, of course. I struggle whether this is a functional or a non functional issue.
The spec says: when the button is pressed and X is not shown, show it. When it is pressed and X is shown, hide it.
It does work like that but in addition, continuous holding of the button causes X to show & hide repeatedly.
I am thinking about non-functional requirement but cannot think of any that would fit.