TYPO3 Core

Method returnFileReference() in t3lib_Compressor tests for browser support before actually allowing a CSS or JS compressed file to be returned to the client:

    // if the client accepts gzip and we can create gzipped files, we give him compressed versions
if ($this->createGzipped && strpos(t3lib_div::getIndpEnv('HTTP_ACCEPT_ENCODING'), 'gzip') !== FALSE) {
    return $filename . '.gzip';
} else {
    return $filename;
}

This works fine from a BE perspective but fails in Frontend because the first client will trigger the caching framework and the result will be cached and reused for all following visits. If the first client supports gzip but the second one not, the site will be broken. If this is the other way around, then the download experience will be suboptimal.