Debugging Migrate: how to print what data's mapped to a field
Answer from Mike Ryan: I usually run migrations in drush and, when not stepping through in a
debugger, use drush_print_r(). The key points you want to instrument are
prepareRow() (dump $row to make sure the source data is being pulled
correctly in the first place) and prepare() (dump $node to make sure the
source values are getting properly mapped into the node object that's about
to be saved).