I'm talking about the free roll game.
Their system is provably fair, of course, but who's so stupid to save and check all the hash of every free roll? Me
My day job allow me to work on a computer so i'm able to roll about 14 - 15 times a day.
However, over the last months i managed to collect about 3000 free rolls, thanks to a script i managed to save every server seed hash, client seed and nonce right before and right after every roll, so i could check them later.

Most of them are fine, but 2 times happened that, after the roll, server seed hash was REPLACED with a new one, while nonce and client seed where still the same. This completely break their fairness, since replacing the server seed hash means that they are replacing the server seed and therefore they are changing our rolled number.

Server seed hash never change unless you roll. you can reload the page, logout and login and it would be still the same.
I'm fully aware of freebitco.in's thread where people post their big wins.
Since the highest number i got was 9993 and since i never deposited any satoshi, my guess is that they apply this cheat just for the first 3 prizes (so from number 9994 to 10000) only to people who have never deposited anything.
I say so also because probability to win one of the first 3 prizes is roughly 6 on 10000, so 2 on 3333 on average and this cheat happened 2 times over 3000, quite similiar to their big win probability.

If freebitco.in want to follow this behavior to discourage people like me who just play free rolls and never deposit, that's totally fine but they should atleast be honest with their users and it should be written somewhere