[im3w1l]

> In consensus systems, however, any difference in behaviour can be a fatal vulnerability.

Another issue can come if you have code like this, and different validations are used.

    if (signature_valid()) {
      // Safe because we already verified signature.
      use_signed_message_or_die();
    }

In practice the check and usage could be very far apart in the code (maybe using the type system to keep track of verified signatures), or even in different programs.