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i've got bug in Opera 11.64 browser. Check it out here http://jsfiddle.net/wqCDm/13/
Button has background: url(data:image/svg+xml;base64... and is located in block with position: fixed. When you start scroll page background rolls away, and after you get back to the top it shows up again.
Without border-radius it works properly.
I confirmed that this is a bug and it is still the case in Opera 12. Did you report it on Opera Bugs Reporting system?
Update 2012-06-21: OK. CONFIRMED. It is now CORE-47072 in Opera Internal Bug tracking system.