This will be a bit weird and hardly reproducable one... :(

When I'm trying to call some global function from
a class, parser occasionally reports
Fatal error: Call to unsupported or undefined function...

It usually happens in large scripts composed from more classes if called with POST method, but I'm sure
"missing" function is always defined - it happens in
scripts like

<?php
function sql_debug($msg) {
    if (!empty($GLOBALS['SQLDEBUG']))
        echo htmlspecialchars($msg);
}

class sql {
   ...
   function query($query) {
       sql_debug($query);
       ....
   }
};
?>

When I resubmit form data, everything works.

I have PHP compiled with MySQL support and with
php-czech patch (http://lide.punknet.cz/miri/php-czech/php-czech-0.30.patch)
but this patch only counverts output of PHP script and has
nothing to do with parser, that's why I don't think it
could cause this problem.