I received this error message:
Fatal error: Call to a member function attributes() on a non-object in \libraries\joomla\installer\adapters\template.php on line 55
when trying to install a 1.5 template through the discover method on a Joomla 2.5 system.
Solution
Edit the templateDetails.xml file and change the <mosinstall tags to <install
<install type="template" version="2.5">
And make sure you change the close tag from </mosinstall> to </install>