The custom SQL syntax suggested in the Symfony Book 1.0 is vulnerable to SQL injection attacks. The concerned section is here: http://www.symfony-project.org/book/1_0/08-Inside-the-Model-Layer#chapter_08_sub_using_raw_sql_queries
Although I have only tested this on Symfony 1.0, I believe all versions of the documentation are incorrect since little has changed in this section of the book in the newer versions.
The following code demonstrates the vulnerability:
$connection = Propel::getConnection();
$query = "SELECT %s as user_id,
%s as email
$query = sprintf($query,
"myuser'; UPDATE user SET email@example.com' WHERE user_id=1 AND username<>'");
$statement = $connection->prepareStatement($query);
$rs = $statement->executeQuery();
This is a serious problem as the Book is used by many as the definitive source of information for both Symfony and Propel. See http://propel.phpdb.org/trac/wiki/Users/Documentation/1.3/Upgrading for examples in Propel 1.2 and 1.3 of how to properly prepare custom SQL queries that are not open to injection attacks (search for "General DB API Changes").