Ground plane are nice for various things. However they also have their culprits. You may do a little search and sure enough you will find some documentation on how to implement them the right way (assuming the author know how to 
)
You want to avoid unconnected islands, those may give you strange results and maybe screw up your whole design.
Take a bit of your time to read those bellow.
http://www.ce-mag.com/ce-mag.com/arc...103CE_028.html
http://www.radioing.com/eengineer/pcb-tips.html
http://www.amd.com/files/connectivit...86cc/22507.pdf
HTH
Steve
It's not a bug, it's a random feature.
There's no problem, only learning opportunities.
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