I've tested this in 1.16.5 but I'd assume that this bug would work in most versions of Minecraft. The Steps go as follows

1. Create a unique banner
2. Place unique banner as well as a base color banner in a crafting table in an attempt to duplicate the design
3. before you take out the duplicated banner, "acidently" shift-click literally any other item in the crafting menu
4. You'll find that the crafting recipe still works!
5. Click the product banner to take it out and notice as one of the item(s) that you put in that should not be involved with the recipe is lost to the void, never to be seen againe