The trouble with surrogate nulls (storing real data and pretending that it represents missing data) is that you never know when the real data is going to be interpreted as real data.
Here’s a twist on the surrogate null issue: a case of shorter SSN numbers from Micronesia being padded with leading zeros - presumably a software requirement – which comes causes all sorts of frustration for those with SSN numbers that actually do begin with zeros.
There are two lessons here:
First, respect data and don’t presume that real data can pretend to represent missing data, even for a few digits, without being found out eventually.
Second, data decisions should be made by a data architect with the experience to anticipate and prevent foolish errors.