Kotaku.com recently got news that the new Crackdown downloadable content has given people problems with losing saved content.
Kotaku said this,
"The problem's arising from co-op games where the host has the Keys to the City cheat mode enabled, which is resetting some people's data. So long as you don't join a co-op game where the host has this mode enabled, you should be fine."
They were quick to note that its just happening to a "lucky few", as they put it.
Plus they mentioned a temporary way to avoid this problem.
For more info, click on the link.