So let's just assume that PrimeDice's scheme is fair (which is is 
) we need to ask "how can they still cheat" and we can come up with a list:
1) They don't credit your deposit
2) They don't process your withdrawal
3) They don't properly adjust your balance according to the bets
4) They give you a maliciously picked client seed (or alter the code that generates them to do so)
5) The bet results don't match what they should (i.e. altered bet amounts / targets / outcome )
) we need to ask "how can they still cheat" and we can come up with a list:1) They don't credit your deposit
2) They don't process your withdrawal
3) They don't properly adjust your balance according to the bets
4) They give you a maliciously picked client seed (or alter the code that generates them to do so)
5) The bet results don't match what they should (i.e. altered bet amounts / targets / outcome )
Those all don't touch a point that very little people realize.
Provably fair means that you can re-create the seed using known hashes.
I could stay here and talk about provably fair all day long, it works, and it stands for something. But the other element that people miss is how a random number is turned into a gambling number. For dice, for roulette, for god knows what kind of gambling system that uses random numbers to generate its bets.
You see, the original number CAN be random, but the way it's converted into a bet would make all the difference, and makes me seriously doubt every gambling website out there.
It's simple :
You generate a true random number using server + client seeds + a nonce that gets incremented. So far so good. But once you do generate the random number, (let's say that has a range of 0 - 1.000.000), instead of having your algorithm use a normal distribution to convert the random number into a percentage, you lie in your odds.
Say every number between 0 and 50.000 will result in a losing bet of 1x, but everything else will increment into a nice exponential looking slope.
This effectively makes the house edge 5%, and goes completely undetected, and unprovable unless you have access to the code that turns a random number into a bet.
This is probably very common because rare are the websites that open-source, and fully client-side their bet verifiers. And even then, you'd need someone with knowledge to go into the source code, verify that there isn't a hidden range that benefits the house more than they should.