This bug is only visual, but if a player starts breaking a block in survival and then is switched to adventure mode, they will still be able to punch that block and make the block break animation play through. No block is actually broken though.

Steps to reproduce:

• Open a world in 15w35b
• Start hitting a block in survival mode, but don't break it.
• Type /gamemode 2
• Start hitting that SAME block

The effect disappears when the player returns to gamemode 0 and starts breaking another block or relogs.

You'll notice the animation cycles through.
Detailed video on this bug by COD3R3D: https://www.youtube.com/watch?v=xywIc4-1yzk

Quick gif showing a player in gamemode 2 trying to break a block: https://i.gyazo.com/b0c09c349ff28fcbba435c6dc37ba531.mp4

Screenshots have also been attached.

Hope this helps!