We currently call consumeBlock unconditionally, then use the returned stack in placeStackAt. If placeStackAt fails to place the item for whatever reason, it gets left in the ItemStack, but we forget to put this back where it came from, so it gets voided.

But can placeStackAt() still fail? Normally before doing consumeBlock the builder will have done various checks to ensure that placement works

Yeah, there's still a couple of edge cases where that can happen. For example, trying to plant sugar cane on a block that isn't next to water.