N/A. No error message(s) occur.
The steps to reproduce this issue though are unique and worth mentioning. Knowing how this occurs can then be avoided. There is also a patch for this issue.
To reproduce the issue, create a new legal hold, Click Add Custodian Notice, click the Add Custodians... button, select and add a few custodians, press OK, click the checkbox Send now and then click Save button. The notifications get sent to those custodians as expected.
Click Add Custodian Notice, click the Delete All button, select and add a new custodian, press OK, Now press the Delete All button again as if you accidentally selected the wrong custodian(s) and need to change, select and add a new custodian, press OK, click the checkbox Send now and then click Save button. The notification will now send to all custodians within that legal hold.
The below screenshot is from a Veritas lab with made up user accounts. Here you can see the first notification was sent to 4 fake user accounts. Then a second custodian notice was created to be sent to one user named Jeremiah as seen in box 1 but the notice was sent to all custodians as seen in box 2. This only occurs following the exact steps mentioned above.

Issue within the eDiscovery code.
Workaround: Avoid adding custodians using the steps mentioned above. If not pressing Delete All button and adding custodians more than one time before saving can avoid the problem.
A supported hotfix has been made available for this issue. Please contact Veritas Technical Support to obtain this fix. This hotfix has not yet gone through any extensive Q&A testing. Consequently, if you are not adversely affected by this problem and have a satisfactory temporary workaround in place, we recommend that you wait for the public release of this hotfix.
Veritas Technologies LLC currently plans to address this issue by way of a patch or hotfix to the current version of the software. Please note that Veritas Technologies LLC reserves the right to remove any fix from the targeted release if it does not pass quality assurance tests. Veritas’ plans are subject to change and any action taken by you based on the above information or your reliance upon the above information is made at your own risk.
Please contact your Veritas Sales representative or the Veritas Sales group for upgrade information including upgrade eligibility to the release containing the resolution for this issue.