Customise errors when validating model data?
In addition to Brad&#8217;s answer (which is what I was looking for), you could also use template-specific messages using conditionals if for some reason you only needed to override the global error in a certain place. Unlikely but here’s an example:

{% if errors.attributeOne is defined ? errors.attributeOne | length %}
    &lt;p class="alert"&gt;Custom message.&lt;/p&gt;
{% endif %}