Be aware that Yahoo aerial imagery is old. Sometimes very old. And cities are changing fast these days. When walking along Zurich-Affoltern I discovered a modern and new residential area. Not even the construction site (several years old already) was present in Yahoo aerial imagery. But people started tracing buildings in this area not long ago; buildings that were demolished years ago.

tracing from aerial imagery is nice for rainy days and when you are out of stuff to do in your own area (beware: there is always something left to map in your area ;) ). But make sure you at least KNOW what you trace and check for the age of the imagery you're using. Using outdated imagery if you haven't any clue what the area looks nowadays won't help anyone.

If you see outdated information delete it, inform the creator if needed or put at least a bug report in openstreetbugs or add a fixme tag. Check with your local or national community and inform them about the status of aerial imagery and development in a certain area if the map and aerial imagery is heavily out of date.

Have fun mapping!