Risks of Not interpreting One-Way Function

New York City officials anonymized license plate data by hashing the individual plate numbers with MD5. (I know, they shouldn't have used MD5, but ignore that for a moment.) Because they didn't attach long random strings to the plate numbers -- i.e., salt -- it was trivially easy to hash all valid license plate numbers and deanonymize all the data.
Of course, this technique is not news.