There was an exploit (it's been resolved), in the call of duty 4 beta. The public beta had 100k valid participants who qualified and were provided special tokens which allowed them to download the COD4 beta from marketplace.
There was an exploit that enabled valid beta participants to sign in with their accounts on a different machine, use their beta token and download the beta. The beta was then fully functional for any LIVE profile on that 2nd machine (even after the beta participant's account was removed). Approximately 23k consoles were used to take advantage of this exploit to illegitimately access the COD4 beta.
The exploit has been fixed to prevent any additional users from illegitimately accessing the COD4 beta.