In core/relationship_api.php, function relationship_get_details, there is a spare unclosed column after the related bug number, on:

if ( $p_html_preview == false ) {
$t_relationship_info_html .= '<td><a href="' . string_get_bug_view_url( $t_related_bug_id ) . '">' . bug_format_id( $t_related_bug_id ) . '</a>' . $t_td;
$t_relationship_info_html .= '<td bgcolor="' . get_status_color( $t_bug->status ) . '">' . $t_status . ' </td>' . $t_td;
}
else {
...
}

Note that the first line has a leading $t_td, and the second line has another <td bgcolor...>.

Since due to this spare column, this table has a total of 6 columns, and below, in the function relationship_get_summary_html, there is a line with uses colspan="5" to display lang_get( 'relationship_warning_blocking_bugs_not_resolved' ). So this line breaks before the right edge of the table, and this was how I found this bug.