Middle clicking an acacia leave in creative if you don't have an acacia / dark oak leave in your inventory will give you a glitched "ghost" acacia leave that cannot be placed and doing anything with it makes it dissapear. It works with naturaly spawned acacia trees and acacia leaves placed by the player / command blocks.
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This issue duplicates MC-86391.
First appeared in version 15w33c.
Confirmed. It creates a stack size of 0 and places whatever block was in that slot before it was replaced by the leaves.
Also, it should be noted that using pick-block on Dark Oak leaves always gives you Acacia Leaves, regardless of circumstances.