If you mine a block in Adventure (only in adventure) and look away before you destroy it, you will see cracks in the block that do not disappear automatically. Not sure if this is intentional, but it's pretty odd.

1. Get a pickaxe or a tool to break a certain block
2. Start mining that block, but don't destroy it.

If you try to very slowly mine it more, you will get to a point where the cracks reset and the block returns to normal. If this ticket is a duplicate or similar to another ticket, please let me know. I could not find this when I searched for it.