If you're really trying to write efficient code (which is what this article claims to care about) you don't 'forget' to do things like assert that the data is correct. You _guarantee_ that the data is correct and then you process it as fast as you can without having to check.
You only run into trouble with this method if the data verification step is the computationally expensive operation.
You only run into trouble with this method if the data verification step is the computationally expensive operation.