The bug

By placing an illager banner and a white banner in the crafting grid, I was able to create a second illager banner.

_{I don't think this is working as intended, because illager banners are otherwise uncraftable in survival. They have seven patterns, while only six are possible in survival without cheats. I had assumed they were intended to be obtainable solely as trophies from illager patrols.}

How to reproduce

Place an illager banner and a white banner in the crafting grid.