Forgotten Weapons has the story on an 1895 Winchester-Lee rifle that exploded three days ago. The rifle was firing ammunition made from modified .30-40 Krag brass at the time of incident. Fortunately, the owner was only mildly injured.
Ian of FW speculates that the .30-40 brass commonly used to create 6mm Lee Navy ammunition might not be strong enough after modification to accept load data for that caliber. I’m of mixed opinion. On the one hand, the original .30-40 Army did have a thinner case web, which could lead to problems when firing ammunition based around the original 6mm performance specification. On the gripping hand, it seems unlikely to me that modern .30-40 Army brass would be so weak as to invite those sorts of problems.
I don’t think it’s early enough to conclude that bad metallurgy is the culprit, but it is important to remember that metallurgy and safety standards both have come a long way since the late 19th century. Of course, modern ammunition – even modern handloads – are also much better, so any handloader following the book when loading for an old rifle should, in theory, be alright.
Pictures of the kB can be found in this imgur album.
Stay safe, everyone.