One of my sites had a funny error complaining about an "unsupported form of compression" shortly after installing a new module. The browser would return a white screen of death with an error message (Safari and Firefox).

Content Encoding Error
The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression.

Turning off the module would have stopped the issue, but I did some googling and noticed a post on Drupal.org about a similar content encoding error message for a different module. The problem ended up being related to the level of PHP error reporting on his server - that is, when an error occurs, PHP will send some text at the top of the page informing you of a problem with your code.

Turns out there was a bug in the code. I went digging through the issue queue for that particular module and noticed another user having the same problem, so I submitted a patch and applied the fix to my local code. With any luck, the next release of the module will contain the fix, and nobody will ever have the exact same error.

The problem ended up being caused by the page compression. Drupal uses page compression techniques to zip the outgoing HTML to make the file size smaller, which is supported by most browsers. The white screen happened when the PHP notice got added to the top of the page. Normally, a PHP fatal error would not send any data, just the error. In this case, the problem was just a notice-level error, so the HTML output was still sent, and somehow the notice text was not compressed.

In order to see the error, I used a command-line tool called wget. Then I used a text editor to inspect the top few lines of the incoming source. Most of it will be garbage, but if you're having the same PHP notice problem, you will see the notice right up at the top.

Here's the line I used for wget:
wget --header "Accept-Encoding: gzip,deflate" http://[site_url]

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