When quickly shoveling dirt, and you hit a tall grass block quickly followed by a dirt block, the dirt block will not break. When you stop mining for a second, and go to break the block again, the block breaks normally. I was able to reproduce this on a number of occasions.
Link to Youtube clip of this bug: http://youtu.be/6iAdsCdqxPY
Environment
Windows Vista, Java Version: 1.6.0_37
Duplicate of MC-2201