Version: 2.69 official (win 64)
How to reproduce: In the dope sheet of the provided .blend file, select some keyframes and grab/duplicate them. Notice that as you slide those keyframes in the dope sheet, weird, impossible f-curves show up in the graph editor.
Once you left click to drop the keyframes, the curve goes back to normal. This bug is a very minor one, as it has no real effect on the animation. It only affects the appearance of the f-curves during transformation.