graphical glitch:stone block pops out of ground when harvesting potatoes

Put the summary of the bug you're having here
playing on a server in 1.8 pre release 3 when harvesting potatoes stone will pop up instead of potatoes for around a second before switching back to potato. i have not tried with other forms of crops.
*What I expected to happen was...:*potatoes pop out of ground
Describe what you thought should happen here
i thought potatoes should pop out of ground
*What actually happened was...:*stone popped up for a moment
Describe what happened here
think its a graphical glitch as i have not had the stone appear in my inventory.

Steps to Reproduce:
1. Plant potatoes
2. ...at any point wheather they are fully grown or not harvest them
3. ...I found the glitch to be around a one in ten chance for stone to be 'harvested' before changing back to potato.