Chris Shiflett points out an unexpected SQL injection possibility as presented by Alexander Andonov for PHP (involving mysql_real_escape_string).

The focus of the article is stressing the importance of filtering input and escaping output, as neither is a substitute for the other, but he does so very clearly with specific examples [...] A number of example exploits are supplied for each case, and he discusses which ones work, which ones don't, and why.

Chris also uses the post to link to Paul Reinheimer's post about add_slashes versus mysql_escape_string and his own post on the same topic.