It's almost unheard of to write a data import that just works on the first try. Our examples have all been written using data that is known to be in good shape, and to be honest we've avoided even trying to import some of the data because it ended up being problematic and we wanted to focus on working with the Migrate module and not debugging the problems in your source data. In the real world though, you're going to end up with problematic rows in your source data and you'll need to get them resolved.
In this lesson we'll run the complete player migration and end up with a couple of rows that fail to import because of an oversight in our code. We can use the migrate UI to get a sense of what is failing and why. Then we'll use a combination of options available to the drush migrate command and some strategically placed print_r's to debug and resolve the problem rows. Finally, we'll use a trick to get the Migrate module to re-import all the problem rows but not the already imported rows.