It is currently not possible to grant additional permissions for a single user to a single issue.

This would be a very useful feature. Consider a JIRA user who has set up several issue security levels for one product, so that different customers of this product can only see their own issues, but not those of the other customers.

It would be very helpful if the JIRA administrator could grant browse permission for one particular issue (belonging to customer A) to one particular person belonging to customer B. The person could then find out whether this information is relevant do him. But he could not see any additional customer A issues, and no other person of customer B has any additional permissions.

Another scenario: a user who is not a developer attaches a patch or some source code to an issue. It would be useful to grant this person some developer related permissions to this issue only, since he is the one currently working on this.

The perfect solution is to have an extra control which works exactly like the existing "Add User/Group to Issue Security Level" or "Add New Permission" control, but which operates on a single issue instead on a security level or a permission scheme.