On the Pumka.net blog, Anton Oliink has an interesting problem where his timestamp on the PHP side of his application was different than the one on his MySQL backend's side - by 24 seconds, in fact.

You may find that timestamp value returned by MySQL UNIX_TIMESTAMP() function is 24 seconds grater than those returned by PHP functions and classes like strtotime(), mktime, DateTime::getTimestamp(), Zend_Date::getTimestamp().

As it turns out, the issue isn't' really an "issue" after all - it's caused by MySQL's compensation for leap seconds. He gives a few ways you can avoid it being an issue in your application, though: disable leap seconds, only convert to timestamps on the PHP side or just use the "unix_timestamp()" and "from_unixtime()" methods to work with the values.