Got leather from fishing (junk, which is fine), but received the Cow Tipper achievement. I had not yet received the Cow Tipper achievement, but I got it from fishing. Since (as far as I know) the achievement only checks for picking up the leather, it can be achieved from finding it in a loot chest dropping it and picking it back up again, fishing, and possibly other ways. It should probably be fixed to only give the achievement after killing a cow. This isn't a very serious bug, but it's still there.