For me, that would have to be my first HD 600, which was destroyed last year in a house fire. I retrieved it from my bedroom afterward, and while it appeared to still function, it stunk so horribly of acrid smoke that it would never be usable again. It was also covered in soot, and the room had gotten hot enough to make the decal transfer on the headband bubble up and partially separate. I tried disassembling the headphone and thoroughly cleaning it, but it was hopeless. As I would discover with several other objects I attempted to salvage, the smoke smell becomes impregnated into plastic (likely because of the extreme heat) and never comes out.
I showed the remains of the headphone to the disaster recovery people so it could be included in the insurance claim, then I watched them throw it on the pile of everything else that was being disposed of. A sad end to the single object that had brought me more enjoyment than any other, before or since.
I replaced it several months later with a new HD 600 (just before the cosmetic redesign, incidentally), though I've never been quite satisfied with the second one. Maybe my taste changed in the time I was without my first pair; maybe the pads on the new one just haven't worn in the way they had for the majority of the time I had my first pair; maybe the new one genuinely has more pronounced upper midrange, but whatever the case, I find my current pair noticeably more shouty and a bit harsher than how I remember the first one sounding (it always had a hint of shoutiness about it, though it seldom bothered me). I wish I still had the first one to compare—I would never wear it for an extended period, but I might brave a few moments with it just to see if it was indeed a unicorn pair (or if my current one is just a bad one).
Anyway, here's the carnage if anybody's interested: