You're careful with your shisha pipe, yes? And you don't let the hot coals fall all over the floor, yes? Well, if you've been less than careful in the past, take a look at this for an example of when things can go very wrong, because a cafe in Willesden set alight:
The ground floor of a coffee shop on Willesden High Road was left partially destroyed and littered with ash after more than 25 fighters spent just an hour trying to save the premises close to Willesden bus garage last week.
London Fire Brigade think the blaze was started by careless disposal of burning shisha charcoals among piles of rubbish in the rear yard of the premises.
via: Coffee shop fire in Willesden blamed on shisha pipes

When you've finished with your charcoals, don't just chuck 'em in the bin. If there's a lesson here, it's "don't be stupid"!