Blocks Placement Undo

You can easily break the last six blocks you have recently placed.
This makes it easy to fix misplaced blocks, particularly smoothstone, obsidian, and glass.
You do not need the appropriate tool to break blocks in this manner.
Some blocks, such as

Clay will not be broken in the silken manner as it is impossible to tell if that's what you actually want. Holding shift while breaking will force a silk touch.