Transaction status FALSE but DB error NULL

I may not be correct, but I suspect it might be because insert_batch will continue to iterate through the data set even if one item in the set results in a failed insert. Subsequent inserts may succeed meaning that $this->myDB->error(); will report no problem after the batch is finished. If you can set up a test where you know the last insert will fail you should be able to test my premise.

I don't see any way to capture the error when it happens short of extending CI_DB_query_builder and overwriting inset_batch().