I'm at office again, now it's the last PB 6 beta, 126.96.36.199-b and the bug is still here.
Write some text in the Notes field.
Select the text and copy it (Ctlr-C, or mouse right-click and Copy)
Open Gmail web interface and make a new mail
Paste the copied text in the new mail's message area
As you can see there is more than I've copied. I don't know what it means, but it seems some technical data present before my text
Windows 7, Google Chrome 188.8.131.524