I think I found what is the latest gotcha with imagescalers:

```
curl -H 'X-Forwarded-Proto: https' -H 'Host: commons.wikimedia.org' 'http://mw1152/w/thumb_handler.php/7/75/John_Bauer-Tyr_and_Fenrir.jpg/180px-John_Bauer-Tyr_and_Fenrir.jpg' -v
* About to connect() to mw1152 port 80 (#0)
*   Trying 10.64.16.132... connected
> GET /w/thumb_handler.php/7/75/John_Bauer-Tyr_and_Fenrir.jpg/180px-John_Bauer-Tyr_and_Fenrir.jpg HTTP/1.1
> User-Agent: curl/7.22.0 (x86_64-pc-linux-gnu) libcurl/7.22.0 OpenSSL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
> Accept: */*
> X-Forwarded-Proto: https
> Host: commons.wikimedia.org
> 
< HTTP/1.1 302 Found
< Date: Wed, 29 Jul 2015 09:56:29 GMT
< Server: Apache
< X-Powered-By: HHVM/3.6.1
< X-Content-Type-Options: nosniff
< Cache-control: no-cache
< Expires: Wed, 29 Jul 2015 21:56:29 GMT
< Vary: X-Forwarded-Proto
< Location: https://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Tyr_and_Fenrir-John_Bauer.jpg/180px-Tyr_and_Fenrir-John_Bauer.jpg
< Transfer-Encoding: chunked
< Content-Type: text/html; charset=utf-8
```

while on non-HHVM ones
```
curl -H 'X-Forwarded-Proto: https' -H 'Host: commons.wikimedia.org' 'http://mw1159/w/thumb_handler.php/7/75/John_Bauer-Tyr_and_Fenrir.jpg/180px-John_Bauer-Tyr_and_Fenrir.jpg' -v
* About to connect() to mw1159 port 80 (#0)
*   Trying 10.64.16.139... connected
> GET /w/thumb_handler.php/7/75/John_Bauer-Tyr_and_Fenrir.jpg/180px-John_Bauer-Tyr_and_Fenrir.jpg HTTP/1.1
> User-Agent: curl/7.22.0 (x86_64-pc-linux-gnu) libcurl/7.22.0 OpenSSL/1.0.1 zlib/1.2.3.4 libidn/1.23 librtmp/2.3
> Accept: */*
> X-Forwarded-Proto: https
> Host: commons.wikimedia.org
> 
< HTTP/1.1 302 Found
< Date: Wed, 29 Jul 2015 09:58:24 GMT
< Server: Apache
< X-Content-Type-Options: nosniff
< Cache-control: no-cache
< Location: https://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Tyr_and_Fenrir-John_Bauer.jpg/180px-Tyr_and_Fenrir-John_Bauer.jpg
< Expires: Wed, 29 Jul 2015 21:58:24 GMT
< Vary: X-Forwarded-Proto
< Content-Length: 0
< Content-Type: text/html
< 
```

In this case we have the same problem we had with 404s: thumb.php doesn't set the content-length, which makes apache assume the transfer-encoding is chunked, which for a 0 sized body makes varnish barf.