Following code will throw 'Zend_Gdata_App_HttpException' exception with message 'Expected response code 200, got 412 Mismatch: etags =' ...

$client = Zend_Gdata_ClientLogin::getHttpClient('liz%40gmail.com', 'password', 'cp');
$gdata = new Zend_Gdata($client);
$gdata->setMajorProtocolVersion(2);

$firstEntryLink = 'http://google.com/m8/feeds/…';
$secondEntryLink = 'http://google.com/m8/feeds/…';

// Retrieve first entry
$firstEntry = $gdata->getEntry($firstEntryLink);
// Edit first entry and update
$firstEntry->setContent($gdata->newContent('Test content'));
$gdata->updateEntry($firstEntry);
// attempt to retrieve another entry will throw the exception
$secondEntry = $gdata->getEntry($secondEntryLink);

Google Api versions affected: 2,3 (version 1 does not support etags)

When $gdata->updateEntry() is called, 'If-Match' header with entry's current etag is set to the http client adapter. When $gdata->getEntry() is called afterwards http client's headers are reset with the following code:

        // Make sure the HTTP client object is 'clean' before making a request
        // In addition to standard headers to reset via resetParameters(),
        // also reset the Slug header
        $this->_httpClient->resetParameters();
        $this->_httpClient->setHeaders('Slug', null);

Though, in fact it doesn't make the HTTP client clean and we should manually reset all headers we don't need:

        // Make sure the HTTP client object is 'clean' before making a request
        // In addition to standard headers to reset via resetParameters(),
        // also reset the Slug and If-Match headers
        $this->_httpClient->resetParameters();
        $this->_httpClient->setHeaders(array('Slug', 'If-Match'));