Okay, that's actually a much easier question. I made a bottle-conditioned plum mead I liked very much. I misplaced a bottle and opened it after six months. It was no longer enjoyable to drink. It was pasteurized (and did not seem infected),
so I suspect oxidation. This is stronger proof than my calculations, though the calculations are ridiculously optimistic as far as what the yeast could do, as I did not add any "penalty" factor for yeast being old and in a high-alcohol medium.
@RPh_Guy Thanks for the links, I'll read those later. I've read that slight oxidation is desirable in wines as well, but the amount is tiny.