Differential Mode has just landed in lafleur, the CPython JIT fuzzer. It runs the same code with and without the JIT, compares the result, and flags any discrepancies.
It does this while mutating the code in a feedback-guided loop, so it evolves the fuzzing scripts trying to find one where the JIT gives wrong results.
This is actually the 2nd time this feature is implemented (now better and more robust): it used to work, broke, and now is back.