This accident and the one in Ireland last April brings home the dangers of instructing, you always have to be on-guard for everything.
As much as instructing is a sort of 'apprenticeship' to the instructor to becoming a more qualified/experienced pilot, it cannot be treated lightly.
Everyone who has instructed will have memories of 'what could have gone wrong'.
May both occupants R.I.P
sympathies to their families
Pv