According to an anonymous Foxconn worker who spoke with China Business, Apple have returned something between five and eight million iPhone's back to Foxconn over quality control issues. The anonymous Foxconn worker said the issues are "due to the appearance of substandard or dysfunctional problems".
If the anonymous Foxconn workers claims are true, this would have cost Foxconn somewhere in the vicinity of $1.6 billion. This is also assuming that the returned iPhone's are thrown away, and aren't repaired or used to make another iPhone. There's no information on whether this applies to certain iPhone's, or how far these quality control issues go.
Then we have to ponder... do these quality control issues affect the new iPhone?