Tube failure -- what would happen in worst case?

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.