A person who monitors transactions and immediately submits a competing transaction with ridiculously high transaction fee (50% of the payoff) may have a chance to induce miners to include his illicit transaction in the new block instead of the legitimate payment.

Maybe illicit CRS can be a problem for some kinds of checks, but I guess you could feed first Linux kernel commits in the 2016 to some randomness extractor to get something that is harder to manipulate freely.