Something weird is happening on the border of chunks. It depends on the plants growing just next to the glitched ones.

My mods directory: pic
Forge 10.13.4.1558-1.7.10

.. also Iron Seeds? not Iron Plant/Crop?

Makes two of us...

I have actually noticed this as well, but it's extremely rare and I can't reliably reproduce it.

.. also Iron Seeds? not Iron Plant/Crop?

Go look at Wheat :p I swear it used to say Wheat Seeds...

I had the same issue with my blocks, a single tessellator.setBrightness(block.getMixedBrightnessForBlock(world, x, y, z)) call in my rendering code fixed it. This prevents leaking of brightness setting form neighbouring blocks.

Thanks! Here's hoping that fixes it. I know absolutely nothing about rendering...