Sample code:

ItemStack stack = new Dye(DyeColor.LIME).toItemStack()

Trying to give this stack to the player or dropping it into the world leads to nothing happening. It took me a while to figure out why it wasn't working, but it's because toItemStack() creates a stack with an amount of 0.

Changing the code to use the method which takes an amount fixed my problem:

ItemStack stack = new Dye(DyeColor.LIME).toItemStack(1)

This might not be a bug, but I expected the first method to create a stack of 1, which I feel is a reasonable thing to expect. Even if this can't be changed, I feel like it should at least be mentioned in the Javadoc as a caution to it being a stack of 0 and not 1.