Why has the wrong event confirmation letter been sent?

Usually, this will happen because you have not assigned the intended confirmation template to the event. To check this, simply go into the event (either from the Event Planner or Manage Events) and on the 'Details' tab, on the first page, scroll down until you get to this section 'Confirmation Email Template':
 

If the wrong template is selected, choose an alternative one and then save your changes. The updated selection will be immediately available the next time you go into the event after this. 

Please note - If you have more than one Event Confirmation set up in Administration > Email Templates, then you may wish to check which one is set as the default template, as this will automatically select the default when you create an event. 

The below checkbox will be ticked if it has been set as the default event confirmation template.