AssetBundle Load Error

Sometimes when I load AssetBundle, an error and shows
“AssetBundle loading failed because the xxx script serialization hash does not match. Supported: 27a961dd, loading: b11d58c3”
When it does, by repackaging the AssetBundle can solved this error, but I want to ask, does anyone know where this “script serialization hash code (27a961dd)” came from?

I use Unity4.6.8, thanks for your trouble.

After building asset bundle you cannot change the scripts that are dependent to that bundle.