When a transaction is rollback, $DB->print_debug() function is called. This function is printing some "warning" text.

Problem:
In the case of the SOAP/XMLRPC Zend webservice, the servers eval the code returned by external functions. When these DB rolbback warning messages are passed to eval(), it breaks the web services.

A solution could be to set the $DB in web service mode (new attribut to the DB class) and then the print_debug function would not return message if the attribut is set to true.

Cheers.