When a Player or Command Block uses @a, @e, or a UUID, the command returns a successful message but isn't properly executed. Examples:
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).
Even if the player has items, the command will say there were no items to remove.
Will indicate that has given 30 levels to the players, however no levels will be received.
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.
1. Open a world
and note result.
and compare result to that of #2.