Bingo ! Failure of a power unit.
(2) The low flying prohibitions
(a) Failure of power unit
An aircraft shall not be flown below such height as would enable it, in the event
of a power unit failure, to make an emergency landing without causing danger to
persons or property on the surface.
Everyone remembers the 500ft rule and the 1000ft rule, but because this one doesn't have a snappy title. it tends to get forgotten. Yet it's the first in the list of rules, and it's the one that carries the least exemptions.