WWW download an old version of a file

I met a very strange thing. WWW download an old version of a file.
for example.
www = new WWW(“http://192.168.2.70/version.txt”);
yield return www;

but the “version.txt”'s content is old.

it usually happens after I restart the crashed app(when I download an assetbundle not suit my app, it crashed).

Has anyone met this problem? Incredible.
My Unity is 4.5.5f1

so, I have an idea, change the url to “http://192.168.2.70/version.txt?rand=”+Random.value, it worked.