The problem is the border-top of the buttons. I just added
border-top: solid 2px transparent; to .button
Also, for what it's worth, you can simplify that css quite a bit. You've got quite a few attributes that get repeated for many styles. You should put them into a style that they all inherit from, much like you have with (for example) the border-radius on the buttons.
Another observation - you really shouldn't be using those 5 spaces between buttons to position them. There's lots more that could be improved here, but I'll leave that as an exercise for the reader.
Here's a slightly more concise version:
http://jsfiddle.net/hnLEG/1/[
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