Working with maps i have an activity which is launched when no connection is available and uses offline maps (MapQuest). The activity runs fine, map is shown, and all overlays,
I was having the exact same problem... thanks to you pointing out that it is caused by (likely) when the zoom controls are still visible. I tested it, and that was correct. When I pressed the back button with the zoom controls showing, it would show that leak error, if I waited until the controls faded away (they do after you stop scrolling), then there was no leak error.
A little research in WebSettings provided a method that doesn't show the zoom controls, which means it doesn't leak at anytime you want to press the back button. It does still zoom with the pinch effect though. The only disadvantage to using this method is that your controls won't show. But to me, that's worth it, since most users know about pinch zoom for all apps.
Here is what I used:
// make sure your pinch zoom is enabled
webView.getSettings().setBuiltInZoomControls(true);
// don't show the zoom controls
webView.getSettings().setDisplayZoomControls(false);