I wanted to replicate a red banner with a white Thing Banner Pattern design (Mojang logo) so I made an empty red banner and placed them in the crafting table. All is good.
Accidentally, I put my Netherite sword in the crafting grid and noticed "Huh, I can still make the banner even with a sword in the grid. Neat." I proceed to also place a stack of 16 eggs in the grid as a test, still doesn't make the banner "uncraftable".
I pull the banner from the crafting table and I now have two identical banners, but the sword and one egg are nowhere to be seen. (15 eggs remained in the grid) They were completely deleted, absorbed into the banner.
I expected it to work like how milk buckets work when crafting a cake, or honey bottles with honey blocks: the item remains in the grid and isn't deleted. But alas.
Duplicate of MC-170443.