If you have an fixed element which is curiously positioned relative to it´s parent (what is definitely against the specification „An element with a fixed position is positioned relative to the browser window, and will not move even if the window is scrolled – http://www.w3schools.com/css/css_positioning.asp“), you probably have something done with the transform-property on any of the parent elements.
This will cause this totally absurd behavoir (in every major browser, so maybe the transform specification sucks, or the w3c has some.. reason to do this kind of sh** … whatever).
Here is an example of the „bug“: http://codepen.io/anon/pen/gpPVZV
Thanks a lot to Mr. Blairhippo you saved my day.