When destroying a grass under a 1-block tall grass block the tall grass will stay for as long as you don't turn your head away. NOTE: this bug is present in previous versions and is unresolved. I never noticed until today when I was landscaping and was destroying dirt blocks to save time. I noticed that none of the tall grass blocks were not being destroyed after the supporting blocks were taken away. This is a more in depth look at a previous bug I know this is a dup but I was not able to notice the grass blocks as much so I outlined the bug in an orange outline. You are able to see that there is no supporting blocks under the tall grass. Here is a snapshot of snapshot 1.8-pre 3 with orange outline for visibility of the bug. This is a dup but I was more in depth of what lead up to the glitch. This bug is simple to reproduce:
Step 1: Load a survival world in 1.8-pre 3.
Step 2: Find tall grass.
Step 3: Destroy the supporting block.
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Simple, right? I hope you are able to see first-hand this bug in action and that this will soon be fixed. This bug is present in 1.4.2 and possibly all the way through to 1.8-pre 3. Look for MC-1859 and it's dup's for the original story. There is a way to see if these blocks are real blocks, turn 180 degrees from the block and then look back. This appears to be in a line of bugs involving plant's and tree generation. Look at net.minecraft.blocks, net.minecraft.world.gen for more information. This also results in a chunk error. Also look in net.minecraft.chunk for information.