Sometimes when scheduling a campaign you may get a warning like this:
Templates usually have editable sections in which you can easily modify content.
An editable section added to the HTML of the template looks typically looks something like this:
This is the default content.
Normally an editable section of a template should never be the default since that is usually placeholder text (in the above example: "This is the default content."). SendCastle will give you a warning when scheduling if an editable is empty or unmodified.
In some cases you may be okay with the content being its default, in others not -- it's up to you.