Initially I discovered this when using spreadplayers to move an entity to a chunk that was yet to be loaded; but it appears this behaviour is the same even if the chunks are loaded.
I've confirmed this in 15w47c, and it should be reproducible. I've tested the following using the server console on linux and in singleplayer in Windows:
Create world, seed -1734841898848559871
/summon Pig 152 250 252 {CustomName:Bob,CustomNameVisible:1,NoAI:1,PersistenceRequired:1}
[21:37:15] [Server thread/INFO]: Object successfully summoned
/spreadplayers -200 -168 0 1 false @e[name=Bob]
[21:10:26] [Server thread/INFO]: Spreading 1 players 1.0 blocks around -199.5,-167.5 (min 0.0 blocks apart)
[21:10:26] [Server thread/INFO]: Successfully spread 1 players around -199.5,-167.5
/spreadplayers -200 -152 0 1 false @e[name=Bob]
[21:10:36] [Server thread/INFO]: Spreading 1 players 1.0 blocks around -199.5,-151.5 (min 0.0 blocks apart)
[21:10:36] [Server thread/INFO]: Could not spread 1 players around -199.5,-151.5 (too many players for space - try using spread of at most 340282346638528860000000000000000000000.00)
OK, I admit to blindly typing commands. I checked the area out, and it's deep water. So this is probably expected behaviour, though a more useful error message would be nice.