[SalesForce] Formula field with an hyperlink on the email template

I encountered the same problem today:

System.DmlException: Update failed. First exception on row 0 with id 006o0000003SSs7AAG; first error: INSUFFICIENT_ACCESS_OR_READONLY, user does not have access to use approval assignment email template: []

While the symptom was indeed a Visualforce Email Template, the true cause was that the running user context was the Site Guest User when firing the Workflow Rule.

It is not possible to send emails on behalf of Site Guest User. My workaround in this case was to change the Workflow Rule Email Action to a Create Task Action, which guests are permitted to own.

Visualforce tries really hard to not allow potential security issues. One of the things that is included in this is not allowing HTML to be inserted from merge fields, unless it's part of an <apex:outputPanel escape="false"> element.

Really though a better solution would be to use conditionally rendered elements here. Something like:

Example

<apex:outputLink rendered="{!relatedTo.sample__c = true}" value="http://www.google.com">
      Google
</apex:outputLink>
<apex:outputText rendered="{!relatedTo.sample__c != true}">Value if False<apex:outputText>

The rendered attribute controls if the element is displayed or not, and since they have mutually exclusive values only one of the two will ever be rendered.