In emails that get sent out when the assignee is changed back to unassigned, the following html appears in the email:

<snip>
<table cellpadding=1 cellspacing=0 border=0 bgcolor=bbbbbb width=100% align="center"><tr><td>
<table cellpadding=4 cellspacing=0 border=0 width=100% bgcolor=ffffff>
<tr bgcolor=f0f0f0><td>
The following ticket has been re-assigned.</td></tr>
<tr bgcolor=ffffff><td>
<p>
<b>Assignee:</b> $18n.getText("common.concepts.unassigned")
<br>

</p>

</td></tr></table>
</td></tr></table>
<snip>

To reproduce:
1. Create a new issue.
2. Assign the issue to somebody (you'll receive an email with the proper assignee listed)
3. Unassign the issue.
4. You'll now receive an email with the above html.