Over the few years I've been collecting lights, mainly streetlights and preheat fluorescents, I've had 3 ballast failures but the failure modes of each ballast was different.
The first one did absolutely nothing. No 50hz buzz, sparking, popping, smokey smell or signs of arcing.  (50w MV)
The second one made the bulb flicker for about a couple of seconds before making a brief popping sound and cutting out. (50w MV)
The third one started fine for a split second then sparks began to fly until the breaker tripped. (80w MV)

I'm interested as to why 3 ballasts (2 50w MV & 1 80w MV) would have 3 different failure modes ranging from nothing at all, a brief pop and a fireworks display.
I did notice that the steel laminations on second and third ballasts had started to separate. All 3 had burn marks of varying degrees and showed signs of getting very hot at some point in their life.

Thecore lamination split means there likely was quite some rusting of the core. And the bad habit of rust is, it occupies greater volume than the original steel, so the core expands. That expansion then puts strain on the winding.
And the winding may respond by few ways:
Either the wire breaks open, then it just "dies peacefuly"
Or the insulation layer get pinched, causing short circuit between turns. Consequence of this is significant increase of losses and the winding slowly burning up. This then results into complete burnup. That may end up into a smokebomb, fireworks or just a thermal cutout activation (if equipped)
Or it may cause a short circuit to the core metal, then it usually ends in fireworks and/or tripping the breaker (or a RCD device).