Greg Beaver points out an obscure problem he came across when working with the new _HALT_COMPILER token in PHP 5:

A quick note about an obscure problem peculiar to PHP files that make use of PHP 5's new __HALT_COMPILER(); token. Prior to PHP 5.2.2, if you included two files that contained the __HALT_COMPILER(); token, you would get a notice that __COMPILER_HALT_OFFSET__ was already defined.

This posed a problem for him as he needed it to throw this error for the phar library format. His solution? To patch his installation (PHP5, the PHP6 patch is here) to make the problem a non-issue.