Solve the halting problem, $300-$1000
Unfortunately, the person who put up the project did not specify that the solution has to complete within a timespan. So given a piece of code, a proposed solution could just try all possible values, trying to discover if it will halt. This may take very long.<p>If the project client complains, he will need to prove that your solution is wrong, and he cannot prove that if he does not give your solution enough time to try all possible combinations.