I found a bug "Fatal error: Cannot use object of type stdClass as array" It triggers whenever the cache expires.
This may be due to the fact that I am calling StormTwitter.class.php directly and bypassing all the other bits of the plugin, so it may be due to my misuse of the plugin. However I don't know for sure that it's not a legit bug and I'm not going to investigate further.
To reproduce the bug, on a non-live site, set your cache expiry to a few seconds and then refresh the page where your script is called.
I've posted a solution in the support section.