Shortly after Halo: Reach was released, users found a way to earn cheap, easy in-game credits via an exploit. Some people, though, had to take it too far.
Now, the exploit itself, if you've got the patience for it, that's fine. But 15,000 players have been found to have gone overboard on the exploit via "network manipulation and other easily detectable workarounds that fall well outside of standard gameplay", and every single one of them has tonight had their credit levels reset to a big fat zero, along with receiving a one-day ban on amassing any more.
What's impressive/sad about that number is that it's only from a preliminary pass; Bungie says that "as our automated Banhammer mechanisms grow accurate enough to satisfy our high bar for burden of proof", we can expect that tally to rise.