I have seen this issue and I thought try to figure out what's going on, is this a bug or a somekind of security feature that this page invented (sorry this shows up in one of the application and am not able to get you the direct url, so I thought capture it -- but I can save the response if you want). Thx.
http://screencast.com/t/zimnTCx2V
5 Answers, 1 is accepted
thanks a bunch!
Eric Lawrence
Telerik
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Hey, Eric
I'm having a similar problem with a specific website. The web request is proprietary (API) and includes client credentials so I can't really post it here.
When I send the request to the url via a .NET application, it communicates fine, receives the result and displays it to me without any issue. However when I attempt to view the response data through Fiddler, I am prompted with the following message:
"Fiddler: UnGZip failed
The content could not be decompressed.
The magic number in GZip header is not correct. Make sure you are passing in a GZip stream"
This is on latest version of Fiddler (4.6.2.32002).
I have talked to the developer of the website, and they have suggested they are setting some headers improperly, but aren't sure how to fix on their end. I'm not sure what the issue is exactly. Is there a way to "relax" the error checking on the GZip in the same way that allows .NET and web browsers to load the content fine without any errors?
Thanks!
~ Kurt

The content could not be decompressed.
The magic number in GZip header is not correct. make sure you passing ini a GZip stream.
Any solution of this problem ?
It looks like this may be something with the headers or compression on the server. I recommend using Gzip Test to verify that everything is properly configured.
Please give this a try and let me know the results. Thank you and I look forward to your reply.
Regards,
Eric R | Technical Support Engineer
Progress Telerik

Any news on this?
The error "the magic number in the GZIP header is incorrect" is still present for requests. gzip ist correct.
But if I drag and drop it to the composer, it works.
What ist the difference? Can I call the composer decompress/decode functions within fiddler script?
Thanks in advance!
doc
Hey Doc BT,
Would it be possible to share the session that contains the request with the failing GZIP?