When an issue is created (using full browser CreateIssueDetails.jspa and not using the pop-up screen) and and mandatory fields do not have a value, then the following error message comes:

summary: You must specify a summary of the issue.
customfield_10300: Project Number is required.

It is not necessary to give the field_id (eg: summary and customfield_10300) because:

1. since the field_id is not visible to the end user anywhere else, and
2. the field_ids are not given in the messages below the fields or in the pop-up create issue form.

How to reproduce

1. Create a custom field
2. Add the custom field to a screen
3. Make the custom field 'required' or mandatory for the project and screen.
4. Create a new issue using the full screen form (not the pop-up form)
5. Do not enter the mandatory fields (summary and custom field) and press 'Create'
6. The error message with the field ids 'summary' and 'customfield_nnn' will be displayed.