Have about 51 or more operations. You can have them in all possible combinations (send to group, send to user, remote command etc), but most likely the problem will occur with one type as well. The error visible after pressing "new" and the sub-form no longer opens.

For trunk the error may look like this:
Undefined index: hostid [actionconf.php:771 → CView->render() → include() → getActionOperationDescriptions() in include\actions.inc.php:428]
Undefined index: hostid [actionconf.php:771 → CView->render() → include() → getActionOperationDescriptions() in include\actions.inc.php:621]

For 4.0 the problem may occur in different place:
Undefined index: command [actionconf.php:771 → CView->render() → include() → getActionOperationHints() in include\actions.inc.php:919]

This happens due to array of data being too large. This is max_input_vars in php.ini.