Debugging (Resource) Model Instantiation
Follow each step manually and any bugs during (resource) model instantiation will become obvious.
Since many people find model and resource model instantiation to be one of the more challenging things in Magento 1, these are all the steps Magento does to resolve the factory name to the real PHP class name.
To debug, simply follow each step manually until you find a non-match. This works really well in my experience.
Less guessing, more and faster results.
In the examples I use a factory name of "example/thing".
This is split by Magento to a "class group" "example and a part-after-the-slash "thing".