"If you spend the night in front of the screen, your head will hurt."
Translation:Si tu passes la nuit devant l'écran, tu auras mal à la tête.
It does seem to be an unlikely sentence though. It is not the kind of thing that is likely to be said to someone who you vouvoyer, and spending the whole night in front of a screen tends to be a solitary activity, so it is not likely to be plural either.
It may have been a deliberate choice to disallow it. (But they give the impression that they get it wrong by accident so frequently that it's anybody's guess!).
IMHO, the prudent translator would choose "tu".