So in the past Microsoft have often messed up game pricing and region locking which resulted in free games including things like Assassins Creed III but recently they have had a number of regional glitches (including Costa Rica / Oman / Bosnia) which allowed people to grab games like Injustice and Rayman for 'free' (clicking the appropriate games info page would show the game as 'free' and you could download it for free to your Xbox).
I posted one of the glitches around this here :
viewtopic.php?p=783946#p783946In the past they have never taken any action against people doing this , they simply fixed the system and moved on, however this time it seems they are , there have been sporadic posts on CAG / Neogaf / trueachievements of people who used these glitches getting their consoles banned , their 'out of region accounts' banned and if they played the games on them - their main accounts banned (talking at the moment of maybe around 10 people) - the wording used is 'marketplace fraud' and that this was against their terms of use.
Its not everyone , but there are enough of these happening to make it seem like they are either making an example of some people to deter everyone else or are just going to take their time and slowly go through people who used these glitches and ban them.
If they ban the console you will see it when it turns on - you can still access the content on the local machine but will not be able to connect to Xbox live so no multiplayer gaming , no achievements , no 'apps' , no chance to re-download content , no chance to purchase new content on that box.
If they ban your main account then the same as above but you cannot use your account on any other systems and will lose any money / credit you have on your account (along with your gamerscore)
Most of the threads that I've seen in other places revolve around some very silly analogies (stealing a car) or arguments over whether downloading these free games were 'morally right' or not
tldr - If you grabbed one of the free games and have not played it on your main account dont - wait and see what happens.