@a, @e, and any UUID references work incorrectly after a player dies.

Summary:
When a Player or Command Block uses @a, @e, or a UUID, the command returns a successful message but isn't properly executed. Examples:

/give @a minecraft:stone

Chat messages output to indicate items were given, however it drops them near world spawn, or at the spawn point of the player (this is something I haven't properly tested).

/clear @a

Even if the player has items, the command will say there were no items to remove.

/xp 30L @a

Will indicate that has given 30 levels to the players, however no levels will be received.

/gamemode 1 @a

Assumed from survival. Hearts will disappear, etc. and the client will be able to fly, however they will be kicked if they fly for to long.

Reproduction:

1. Open a world
2. Type

/give @a minecraft:stone

and note result.
3. Type

/kill

.
4. Type

/give @a minecraft:stone

and compare result to that of #2.