Not alarms true but can happen anywhere. Some friends of mine build and install motorised Security gates. They were on a job yesterday where the supply cable for their gates were installed by a local "Electrician" but had to be moved as it was on the wrong side of the gate, they were told that it was connected to the garage fusebox and isolated.
Where the cable came out of the ground there was about 500mm sticking up with bare ends. So they drill a hole through the concrete post and without bothering to check that the cable was dead (They have the kit to do so and have been taught how to use it), they go to feed it through the post. One almighty bang & a load of sparks later, they discover that the cable was already connected to a 16a breaker in the garage consumer unit & this was turned on.
Now the Electrician was in the wrong for leaving the cable live and the ends sticking sticking out bare, but the gate fitters complacency over safety could have got one of them killed.
Always check & double check that a mains cable is dead before touching it people, if your in any doubt then call a Decent Electrician.