I have a contribution page that's used to solicit in-kind donations. On the Amounts tab, Execute real-time monetary transactions is disabled, no Price Set is selected, but the Allow other amounts box is checked.
On attempting to submit this form, I get this notice:
messageNotice: Undefined variable: contribution in CRM_Contribute_BAO_Contribution_Utils::processConfirm() (line 378 of /srv/buildkit/build/d46/sites/all/modules/civicrm/CRM/Contribute/BAO/Contribution/Utils.php).
And no contribution is saved. This behavior is replicable on the 4.6 sandbox (http://d46.demo.civicrm.org).
Regarding the error message, $contribution is not set because $form->_contributeMode is null.