When a mob is spawned in riding another mob and you change certain NBT of the mob on the bottom both mobs will freeze and then a few seconds later the one on the bottom will despawn or die.
The mob that was riding the other one will remain still, if the NBT changes while the 2 mobs are falling through the air they will float as if the NoAI tag has been used.
An example of this is summon a Spider Jockey into the game.
/summon Skeleton ~ ~10 ~ {Equipment:[{id:bow},{},{},{},{}],Riding:{id:"Spider"}}
and then run this command:
/entitydata @e[type=Spider] {Equipment:[{},{id:minecraft:bone,Count:1b}],DropChances:[0.0F,1.0F]}
If the bottom command is applied to a normal spider where nothing is riding it then everything is fine but you apply it on a spider (or any mob) with something riding it an the issue occurs.
Hope i've made the issue clear, wasn't sure how to word it.
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[In response to Searge]
"...modifying internal NBT tags during the lifetime of an entity..."
...is a feature now that /entitydata is in vanilla, therefor any issues should be looked into. Saying it's not a bug is just as silly as saying that issues with switching gamemode are invalid, even though there is a /gamemode command.

In that case it would be easier for us to remove that command again. This is a bug tracker, not a discussion forum.
Problems related to modifying internal NBT tags during the lifetime of an entity are not considered bugs, they will be marked as "working as intended",