The lava texture is missing while flowing.
And only one side don't have "missing texture" text if is a source.
If you can't produce the bug, use any texturepack for see it.
Details: I use a texture pack with only items are custom, the blocks folder is empty.
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duplicates 1
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Quote of the changelog of 13w09a:
Animated blocks and items in texture packs now requires the animation definition (previously it was optional)
Therefore: Duplicate of MC-10625
Cannot reproduce.