How does this even happen. We only have a couple of handsets and you would think Microsoft would test on all firmware configurations.

The only reason is affecting only 10% of the handsets is because they havn't pushed this out to everyone.  As far as I know no carrier branded phone has got the update. Imagine if AT&T allowed the update, how many Samsung Focus units would've been affected. We would be talking 50% or more failure rate.