Parents who sit in their idling cars for (fucking) ages while their cars are facing the tennis courts thus blinding the player on the other side of the court for however long it takes them to either turn their car off, drive off, or someone to tell them turn their fucking headlights off.
Before that I've not had to intervene at all, as far as I can remember.
There aren't that many courts where cars park facing them, but my home courts are one of them ;)
I'd still prefer to override both on/off though.
When the oncoming cars do not have headlights on I find it easier to give them just enough attention to see that they are behaving normally leaving more attention to devote to things other than oncoming cars.
Especially the cars with the projectors that bounce around.