Hello,
This is typically a button that has (partially) transparent pixels, and they cannot be displayed with a Low slide quality (8-bit). I think you are using 6.0.1.240 in which Low was the default slide quality (has changed to Optimized in CP6.1). Please change the slide quality to Optimized (my choice and now also CP's choice) or 24-bit and you'll see hopefully a nice button,
Lilybiri