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
Create a unique banner
Place unique banner as well as a base color banner in a crafting table in an attempt to duplicate the design
before you take out the duplicated banner, "acidently" shift-click literally any other item in the crafting menu
You'll find that the crafting recipe still works!
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
This issue is already being tracked at MC-170443.