I think you misunderstood my point. I am not saying that you should reject valid (that is: semantically meaningful) input, but that if you are confronted with semantically meaningless input, you should reject it rather than garble it so that it gains some random meaning.

So:

Name input field, value "J. O'Brien": accept

JSON parameter, value "{foo:bar}": reject

The context was the idea that you should gracefully accept bad input. If your code considers "J. O'Brien" bad input for a name, then that's the problem, not that it doesn't accept bad input.