When you set your video settings to 'fast' and poke your head inside oak leaves while you are riding a horse, you can see through all blocks that are touching the leaves.
I haven't tested this on other leaves. Look at the files that i have added for pictures.
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duplicates 1
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Oh. I'm sorry! I've been asking a lot of other Minecrafters and nooone knew of it. I didn't know this was a known glitch.
Duplicate of MC-477.