in preview mode if Selection filed is empty (ezselection) this give an error:

Key "0" does not exist as the array is empty in EzPublishCoreBundle::content_fields.html.twig at line 170
500 Internal Server Error - Twig_Error_Runtime

Frontend doesnt output the error using {{ content.getFieldValue('SELECTION-FILED-NAME') }}

Error demo at:
http://www.screencast.com/t/lV6J9ieLlDz

fixing the Problem adding following condition in :
/ezpublish-kernel/eZ/Bundle/EzPublishCoreBundle/Resources/views/content_fields.html.twig
Line 170.

{% elseif not fieldSettings.isMultiple %}

{% elseif not fieldSettings.isMultiple and field.value.selection.0 is defined %}