Traveling north from Whiterun to Solitude I happen upon Elder's Blood Peak. I slay a Blood Dragon there and when it dies, for some reason, I don't receive its soul. Not only that, but its flesh doesn't disintegrate. It just flops to the ground, wholly intact. Whatever, I carry on.
Coming back from Solitude to Whiterun I take the same route, but something's different this time. The Blood Dragon's corpse is gone, and an animate skeletal dragon is sitting on the word wall as a normal dragon would. At first I assume it won't respond since it's probably some sort of spawning glitch (like when dead enemies stand upright and allow you to pass through them), but no, when I draw near he takes to the sky and a fight ensues. I slay him and still no soul. The glitch was so weird that it felt scripted, but I know it wasn't. I was playing on PS3 at the time and have never had it happen to me since.
Must have fought a weird, soulless dragon necromancer.