Unity4.2.1
When I call AssetBundle.Unload(false) to release memory.
Error:
AssetBundle.Unload(false) shouldn’t be called because used assets might have to be reloaded at arbitrary time.If no assets are used, call AssetBundle.Unload(true).
Doesn’t that error explain what you did wrong? 
In WiP8 this method can not be used?
But in the 4.2.0, IOS and Android, this method can be used
Yes, in WP8 that method cannot be used with “false” argument. This is due to operating system limitation. We might have to reload all assets from bundles at any point in time, as D3D context gets flushed when user presses start menu key or locks the phone. That’s why you cannot unload asset bundles when they are being used in scene on Windows Phone.
I see. I will wp8 redesign set of resources scheme.
Thank you
Does this problem still exist on windows phone 8.1?
Windows Phone 8.1 doesn’t have this problem - the requirement to reload textures at application resume has been lifted by Microsoft.
So AssetBundle.Unload(false) will work correct on Windows Phone 8.1?
Yes, it works just fine on apps targeting WP8.1.
After to 4.6.6 version I’ve been getting this error after calling Unload(False). I upgraded to 4.6.8 but doesn’t work. Any update?
Nope. This particular issue will never be fixed for Windows Phone 8.0. As I said, it does not affect games that target Windows Phone 8.1 SDK.