Depends on who the retailer is.
Sometimes, in the case of OCR conversion, it's the publisher's fault.
Sometimes, as a publisher, even when you've done everything right, it's erroneous because of conversion from the retailer. That's why a lot don't like Smashwords--the "meatgrinder" conversion it does could lead to conversion errors.
And sometimes, the retailer doesn't give you tools to test the output. Amazon, for example, in this case:
http://blog.ksaugustin.com/2011/10/0...e-third-world/