So if you're having odd issues with a JavaScript function like WebForm_InitCallback(); which is a built in ASP.NET JavaScript function and you are using the HTTP Compression Handler from blowery and you are testing a site on the ASP.NET development server, I may have a fix for you. 

In your web.config you should add an exclude statement to exclude the WebResource.axd path:

        <httpCompress preferredAlgorithm="gzip" compressionLevel="high">
            <excludedMimeTypes>
                <add type="image/jpeg" />
                <add type="image/gif" />
                <add type="image/png" />
            </excludedMimeTypes>
            <excludedPaths>
                <add path="WebResource.axd"/>
            </excludedPaths>
        </httpCompress>