This week’s savings is on atmospheric puzzle game with an award-winning soundtrack for half-off:

http://www.trueachievements.com/Osmos-xbox-360.htm $9.99/£4.99 $4.99/£2.49

Although the game has received several accolades and generally favorable reviews, it has the unfortunate distinction of being a title that contains an unobtainable achievement. The silver lining is that it’s worth 0G, so you could potentially earn up to 200G. Unfortunately, the game won’t be recognized as “complete” without this 12th achievement.

http://www.trueachievements.com/a41020/ltstring-textgt-....htm

Hemisphere Games only planned to have 11 achievements in the first place, but a 12th “dummy” achievement was included without their intent. The developers had recognized there was an issue, and developed a patch. According to Hemisphere, Microsoft passed on pushing it through, which was addressed on their forums in July 2010.

We actually submitted an update to MS for Osmos on GFWL, and they decided it wasn't worth the trouble. So... it's really out of our hands at this point. =P
If you decide you can overlook this issue, Osmos is on sale through April 11th.