Magento models have slightly eccentric camelcasing 'conventions'. In short do not camelcase your model names.

With magento models, you may be tempted to use camelcasing for long class names, for example MyPackage_MyModule_Model_ALongNameForAModel. This will not work as you might expect on different (namely case sensitive) environments.

Your Mage::getModel(mymodule/a_long_name_for_a_model) call will translate to the class loader trying to find A/Long/Name/For/A/Model.php.

Conversely trying to address your model Mage::getModel(mymodule/alongnameforamodel) will see the classloader trying to load Alongnameforamodel.php, and not ALongNameForAModel.php.

On Windows this is fine, on case-sensitive e.g., HFS (Mac) or Unix file-system, this will not work.