Unhandled CiviMail error

Reported on CSOD/ CiviCRM 1.5
No CiviMail sandbox avaliable on which to try and reproduce.

User reports:
Whenever I cancel a mailing at Step 4 (after the test mailing, but before sending to the whole group) it creates an error.
The URL is /civicrm/mailing/send
And in the log I see the attached error message.

Error message:
Type civicrm
Date Friday, March 16, 2007 - 05:59
User Margaux OMalley
Location http://gcir.civicspaceondemand.org/civicrm/mailing/send
Referrer http://gcir.civicspaceondemand.org/civicrm/mailing/send?_qf_Schedule_display=true
Message CRM produced additional debugging information about an issue or error.

error

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] =>
[message] => HTML_QuickForm_Controller: Unhandled action "Cancel" in page "Schedule"
[mode] => 16
[debug_info] =>
[type] => PEAR_Error
[user_info] =>
[to_string] => [pear_error: message="HTML_QuickForm_Controller: Unhandled action "Cancel" in page "Schedule"" code=0 mode=callback callback=CRM_Core_Error::handle prefix="" info=""]
)