At the last Rendezvous I attended, a man's horn exploded. Fortunately there wasn't much powder in it, but it did catch his shirt on fire as well as burning his arm pretty bad. He'll recover, but I expect he will always carry the scars.
His horn had one of those brass pour spouts that had the spring loaded stopper. It was determined that he was checking his flint for spark when a hot shaving most likely flew up that spout, igniting the horn.
More then likely the spout's stopper was not closed all the way. This in my opinion is/and always will be a hazzard type spout with spring stopper as they do get stuck open if regular care isn't performed on them by keeping them clean, and that spring loaded stopper working smoothly and closing completely as it's suppose to do.
I personally would never use one on a powder horn of mine. I feel the wood and horn tip stoppers are the best/safest, provided you do your part by making sure you've put the stopper back in the horn and get in the habbit of sliding that horn away from your side towards your back with each shot you take, or with every checking of the flint for spark.
Now the man that this happened to is a safe shooter. All of us who know him have always had the up-most respect for his safety habbits. In the hot sun while shooting all day long and getting fatigued, any one of us could make the same mistake and it can happen in a heart beat.
I do hope this man returns to our sport and is not put off by what happened to him. At last check, he has no idea what happened or that this even happened to him. I expect there is an element of shock involved.
Just be safe and be sure your powder horn is working right, and it's always plugged when not in use. And take that extra second to slide it away towards your rear when taking your shot, or checking your flint, or capping off for a match, hunting, or plinking outing.