In his latest post Dougal Campbell shares his findings from a bug he was having with a plugin in WordPress. It revolved around the use of mysql or mysqli and errors being thrown to his logs.
The plugin had previously worked fine (it generates a sidebar widget), and I wasn't actively working on my site, so I wasn't really sure when it had quit working. In the course of debugging the problem, I discovered that the plugin was throwing warnings in my PHP error log regarding the mysql_real_escape_string() function. As a quick fix, I simply replaced all of those calls with WordPress' esc_sql() function. Voila, problem fixed.
He was interested in why this worked, though, and went digging in the code. As it turns out, the WordPress code tries to determine which mysql extension you have support for. As it turns out, his installation fit the "mysqli profile" so the "mysql_real_escape_string" wasn't available. To the WordPress users out there, he suggests esc_sql or $wpdb->prepare() instead.