Enabeling the Recaptcha Extension causes unwanted whitespace.
This can result in issues e.g. when you are creating XML Data that does not allow any output before doctype.

The whitespace is introduced by extra newline in the template autoloads:

ezpublish-recaptcha / autoloads / eztemplateautoload.php

A simple fix would be to remove the closing "?>" at the end of file.