One custom field was a 'number field', and the other was a short 'select list' with two items in it. Both custom fields were for a specific project's issues.

When I deleted the project (a testing project, not a real one), the custom fields remained, as seen in the adminstrator's 'View Custom Fields' section. They also claimed to now be associated with a project which had the same name as the custom field, but this was not a real project.