Reproduce:

I wanted to prepare a custom mform where the Cancel button precedes the Submit - it is a classic wizard form:
[ < Cancel ] [ Continue > ]

The Cancel button inherits from Submit button class. But the file submit.php is not properly included so missing class error arises.

The code:

$buttonarray=array();
$buttonarray[] = &$mform->createElement('cancel', 'cancelbutton', get_string('cancel'));
$buttonarray[] = &$mform->createElement('submit', 'submitbutton', get_string('continue'));