Well TECMO and Microsoft did release their Ninja Gaiden II Mission Pack this past Friday as promised, which stealthily infiltrated players' games and killed them. Seems like a very ninja thing to do to me, but apparently users started getting a bit upset when their game started getting corruption errors after unlocking the first new achievement. The pack was pulled, but on top of that the title update that preceded the release is causing freezing during cutscenes. Luckily the folks at the Gamerscore Blog have some good news...
The team believes they have isolated and fixed the Mission Pack problem that some of you reported earlier this morning. They will continue to test it over the weekend and are hoping to get it back onto Marketplace on Monday. If you already downloaded the pack, go ahead and sign into LIVE. Your achievement should update allowing you to continue playing. The freezing issue is still being investigated.
...and some bad news. That's what you get for paying $60 to invite deadly killing machines into your home. More Ninja Gaiden II Missions for Your Pleasure! [Gamerscore Blog via Eurogamer]