<input name="letter" type="ComboBox">
<label>Select a letter</label>
<occurrences minimum="0" maximum="1"/>
<config>
<option value="a">A</option>
<option value="b">B</option>
</config>
<default>one</default>
</input>
Given the above configuration for a ComboBox the content-type would fail to save with a message "Invalid property for content ", because the default value reference is invalid.
It would be nice to add validation to this aspect of the schema and warn, log error etc accordingly.