Depending on the jet, accidental supersonic can be easy/common, or rare. (eg a strike eagle with the older engines won't go super unless you're trying)

How loud was it and how often did you experience it?

F-104 going supersonic at something like 500ft or slightly lower within less than a mile feels like a full body slap which makes the parts of your clothing which are not tight flutter. Anything a few thousands feet up is still very loud but doesn't do the slapping and fluttering anymore. Anything a few 10.000 feet can still be very loud. Though it varies. Don't know on what it depends exactly. Had 4 in row making nice contrails high up, rather soft. Had single ones, also high up, making a contrail, banging loud and sharp, thinking why was that? Those are all memories from the 80ies. Can't remember when I heard the last supersonic boom anymore. Has to be early 90ies. Today I laugh when I read about people calling the emergency services when they heard one ;-)

They did it near London once by in error. One little Typhoon can produce an unbelievably powerful sonic boom. I felt it as a loud crack/thump on my roof. Now, imagine a Concorde going supersonic...

If I recall correctly. The recent examples of Typhoons going super sonic over London were QRA aircraft hammering it down the country to pick up suspicious aircraft. In some of those cases they are allowed do it

It happens in the Lake District and Wales reasonably often. Often the pilot/crew will go out and apologize to a farmer. Sometimes livestock allegedly die, although I'm not sure what the mechanism is.

That mechanism is trying to get compensated for dead live stock and pulling straws. Thunder is a sonic boom. It’s like saying thunder killed a cow.

very loud sounds can panic/shock animals and if it's too much for them, they die.

Well, in that case, when will we start jailing bikers with loud exhausts for the deaths of old people from heart attacks?

I dream of a day when light and noise pollution are taken seriously. Its exhausting walking beside a busy road with the roar of traffic. You see the birds don't even bother to try and sing anymore. They are just silent and isolated until they get further away. Visual pollution from advertising would be great to curb as well.

Many places do regulate the noise levels of exhausts, which seems a bit more appropriate and less punitive than permitting any exhaust and hoping people don't die as a result. The penalties for noncompliance are obviously not as high as jail time, but at least to me, (among other things) there is an element of premeditation or willfulness missing in these deaths.

God that would honestly be a dream come true actually

Is there documented proof of this actually happening?

Rural person here from Wales where sonic booms from military aircraft were moderately common.

Animals panic when they hear the boom: they tend to stampede, fall off cliffs, break legs, trample each other, lambs get separated, break through hedges and into traffic.

The booms were severe enough to occassionally shatter the Royal Doulton in the house, much to my mother's disgust.

Best childhood memories are from doing walks in the Lakes and having jets zip past in the valleys below. Insanely close, too — I distinctly remember seeing the pilot’s visor and making out white text on the side of the aircraft.

Is the Lake District the infamous “Mach loop”?

No - the Mach Loop is in Wales. Although as the jet flys, they're not far off - so it's common to do lowfly training missions that include both. The loop has a number of aviation enthusiasts who reside there. They'll hear jet noise (or listen on the radio...), then call their friends, get out of their houses, and watch / snap pics of the planes.

I've had similar in Wales. I once got the shock of my life when a jet roared probably not more then 100 foot over my head.

Didn't have a decibel meter, but pretty loud, rattled the house and windows. Not as bad as someone using tannerite, though it might just be that the tannerite is a relatively recent experience and the sonic booms were a while ago.