Put this command in a command block and set it up with a redstone clock:
/summon FallingSand ~ ~2 ~ {Time:1,Block:stone,Motion:[-0.6,0.7,0.4]}
Watch and wait and you will see that approximately 1/50 blocks summoned will be in a slightly offset position, even though the command is summoning the same entity.
Is this still an issue in the current Minecraft Snapshot 15w46a or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.