Wheelers usually look like this
but if the motor shuts down they look more like this
It is worse than tripping over while walking as the rider can not
push against anything to try and recover or reduce their rate of descent.
Normally the rider is warned and strongly encouraged to dismount before the battery goes flat but on many wheels battery protection has been prioritised over rider protection which can lead to a shutdown without warning. Wheels designed this way and fitted with low quality batteries can shut down at any time but even with good batteries they become dangerous as the batteries age. A rider is only likely to be aware of a rare and mysterious catastrophic failure on a wheel that was for a long time safe. Early battery replacement with good cells will avoid shutdowns.
The graph shows published test data overlaid with testing data of an aging cell X (black), damaged cells X (grey) and a generic battery with different, damaged cells X (blue). The discharge rate for the X tests was 0.2 amps. The aging cells were still able to sustain a voltage above the tilt back/warning voltage long enough to warn a rider but the damaged cells went from above the warning voltage to below the shutdown voltage too quickly. The generic battery had a shutdown voltage higher than the warning voltage, making it unsuitable for this wheel from new.
A longer version of this summary is published elsewhere.