2 Convertible Marks (Bam) 2000 Bosnia And Hercegovina / Mint Error

I need to be educated.

How did this sort of error happen?

Here's my understanding. As manufactured and supplied to the Mint, bimetallic coin blanks are rather loose - it is the act of striking the blank into a coin that locks the two pieces of metal tightly together. So, as the blank is being fed into the machine, the core can come loose. So in effect, we have a "displaced off-centre core" - the coin as a whole is properly centred in the dies, it's the core that has moved.

May this help you visually understand how this type of error could occur. By the way, the drawing is for 500 Italian lira from the 80s and 90s of the last century. however, the principle of bimetal planset production is almost the same for all bimetal coins