I found a bug in magento 1.9.3.0. I am not sure how to report it properly, since the "Report issue" I found was down for maintenance.
I will post it on these forums in hope that it will reach the right person.
Problem
I make an API request using XML-RPC to sales_order_shipment.info. (http://pastebin.com/4ZR9CCkA)
In the response, I expect to get back arrays for the items and tracks fields.
Instead, I get back the string "Array".
Complete response: http://pastebin.com/y75R3uT7
Code problem
I traced the issue to the Mage_Api_Model_Server_Handler_Abstract class and the processingRow function.
It does not handle arrays of arrays, so in this case inner arrays always get converted to the string "Array".
Solution
I made processingRow handle arrays by making it recursive. I submitted a PR to the magento-mirror github repo with a proposed fix for those who are interested: https://github.com/OpenMage/magento-mirror/pull/36
