The brain generates a very specific signal (based on region and timing) when it detects a grammatical error—and it generates the same signal when you’re listening to a sentence and realize halfway through that you’ve misinterpreted the syntax and need to re-parse it. I think this latter case is actually the real purpose of the signal: every time it triggers, your brain is warning you that you need to stop and check the sentence again even if the meaning is actually unambiguous.