When you pick up an anvil item in an inventory (like a chest etc) there will be a small line on the anvil that doesn't render in correctly, allowing you to see through that part of the anvil to anything in the inventory behind it. Its a 1 pixel wide line 😅

How to reproduce
1. Get an anvil in your inventory
2. Pick up the anvil with your cursor
3. Look closely at the top, front part of the anvil to see the render issue

I had the best luck using GUI scale -2 when reproducing this bug.