Minecraft 14w11b, Minecraft 1.8.1-pre3
I have been running
spreadplayers 0 0 2000 200000 true @a
to spread two players very far out on a fresh map (default world generation) as part of a mini-game. Most times it works, but about 1 time in 20 or 30, one of the players spawns underground, sometimes in a cave, or other times completely encased in blocks, suffocating.
I am unclear if this is related to a client-side latency issue (e.g. after spread-teleporting, you tend to 'fall through the world' until the fresh world generates in your location), or faulty logic in spreadplayers itself. Either way, it is no fun.
(As an aside, I did notice one time when I spawned underground in a cave, that digging directly up was a village on the surface, so possibly surface structures interact with this bug, or maybe that was just a coincidence.)