If someone were to exploit/employ the +EV strategy on your site, then it would appear that they are simply getting lucky. Also there would be nothing that would stop someone from making small amounts of money across multiple accounts so they would each be under the radar.
Based on posts in this thread, it looks like the OP was offered a total of .5 BTC for his disclosure.
there were 2 bugs, 1 for the +EV bug and a second bug that dooglus found that would allow for players to manipulate some bet data to exceed that parameters set by the site, which i thought 1.5 BTC in total would be more than fair for the disclosure and explanation of these, but upon hearing of the second bug, the admin started getting rather hostile in tone and dissuaded me from accepting any sort of deal.
Well, the EV Bug was so easy to find that it was revealed by a player.
Still watching this thing though.
it was a simple rounding error; the site's client allowed the player to choose a multiplier up to 6500x, every number inclusive. however, their % win chance was rounded to the hundredth place (decimal), allowing for a 0.02% win chance on a 6500x roll, when the correct multiplier for such a roll should be around 5000. this is the worse case scenario allowing for a 30% edge, but as dooglus pointed out above, the second bug allows for even more of an edge.