I'd say that in the US Social Security Numbers run neck-and-neck with names for causing problems, though the data is less screwed up. The problem is that they

I'd say that in the US Social Security Numbers run neck-and-neck with names for causing problems, though the data is less screwed up. The problem is that they look simple, unique, and universal so they're an obvious primary key... but only the first of those is close enough for most uses. Duplicate SSNs have been issued often enough that I've run into multiple collisions in a single production DB (and confusing two people on electronic home arrest can be pretty bad). Even more common are people who simply don't have one - religious objectors, non-citizens who haven't acquired them (though some have them), etc. The net result is that people keep using them as primary keys, as join keys, and as unique constraints and then get bitten hard.