Freezing, not booting, certain applications not working, failing to pass stress tests, and other misc problems that the user reports.
When those things happen they just send them back to the distributor without testing further because they're not sure. They tell them that a)it's not their fault and returns the cpu or say b)it's their fault and sends a replacement.
Could be that AMD is blindly replacing CPUs rather than test them like Intel does.
Just to be clear, I am not defending defective and/or unreliable products, just want to make sure the fingers are pointed into the right direction.