Let's say you have a class name Space_Module_Model_LongModelName and you want to reference it as:
Mage::getModel('module/longmodelname');
This seems to work in some development environments, but doesn't seem to work in all environments. Probably has to do with a file system case sensitivity setting.
The error that you get in environments where it doesn't work is that the include() for Space/Module/Model/Longmodelname.php failed.