Tube failure -- what would happen in worst case?

I friend had flames coming out of the top of his power amp do to a tube failure. I'd be more worried about the house burning down than damage to a component.

When I was a kid we had a Philco color tv that my dad bought in 1968. (I loved seeing star trek in color on that tv back then.) It had lots of tubes and the repairman came to visit us often that first year it was under warranty. I remember one time we turned on the tv and we heard crackling sounds and the wall lit up behind the tv. That was exciting. The repairman came out and fixed it once again. I can remember my dad sending me down to the drug store on my bicycle with a basket of tubes from that tv to test them and buy replacements. Every drug store and hardware store had tube testers back then.

Worst case? That so much depends on the design of the circuit in which the tube is being used. In my case I'm burning tubes in extreme DIY amps and preamps. All the tubes, except the output (that's on the drawing board) have current source fed,shunt regulated B+. Should a tube fail, the shunt device would be required to shunt all the bias current. It is not heat sinked for this and will fail. I had a driver tube in my beta Venice (a 3 stage fully differential phono pre from K&K Audio) arc. The driver stage is direct coupled to the input stage which uses MOSFET based current sources as plate loads for the tube half of a tube/FET cascode. That arc caused one of the MOSFETs in the input stage to fail. I had an output power tubes arc on turn on that caused the grid to short to the plate. That smoked the grid-stopper resistor on that tube. Better the resistor than the secondary winding of an expensive IT transformer. For me, trying to design for all failures is impractical. It's much easier for me to just keep on hand the necessary parts to repair an amp in the rare instance a tube does fail.

I had twice tube failures on my Chinese MC10L.
Both times, a fuse and a register sitting next to the tube blew off, and smoke came.
First time it happened a couple of months after I changed the whole four tubes. After that, I learned how to set the bias.

Second time, it happened in an apartment that I moved to. It was a new place and I did not change the bias. It happened a month after the move.

I also had a tube failure for my BAT vk60. It happened when I tried a seemingly bad power tube. A second after powered up, the tube got bright and failed. Also, a fuse and a register blew up. I replaced the register, fuse, and used a new set of power tubes, and then it worked work OK.
I still have both amps.

After these three instances, I became more careful about using tubes and setting bias. And, I feel that it is NOT a big deal if tubes fail. Looks like the registers and fuses do their job to protect other parts of the amp.

For those who want to try tube power/integrated amps, I would suggest to try with cheap amps first before moving to more expensive models.