Steps:
1. Login to JIRA and create a permission scheme.
2. Set browse project permission as "Current Assignee"
3. Please refer to permission screenshot for other permissions.
4. Associate the permission scheme to a issue and create a issue and sub task.
5. Login user can assign issues.
6. Assign the sub task to some other user.

Result:
1. User gets a big error message "It seems that you have tried to perform an operation which you are not permitted to perform." though the issue is currently assigned to the other user.
2. It seems that after assigning the user when JIRA try to show the issue, its unable to do due to permission and give this error message which is misleading.
3. Giving browse permission to all users solve the issue but its important as "Current Assignee" is an option in permission list.

Expected:
1. No error message and scenario should be handled gracefully.