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    Good info here but your two examples are cases that should be unique identifiers but in the wild are not. Both VINs and IMEI numbers are cloned for criminal purposes and guarenteed to trip up a system that expects them to be unique. Commented Nov 23, 2011 at 23:07
  • If criminals are known to clone some identifier then surely that might be a very good reason to enforce the business rule that the attribute must be unique - i.e. not to "trip up" the system but to prevent the said duplicate values from entering the database and thereby red-flag any such attempts as potential fraud or errors requiring further investigation. Commented Jan 5, 2015 at 12:49
  • What if user makes a typo when entering VIN or IMEI and that typo will need to be corrected? Natural keys should never be used, period. Commented Apr 26, 2021 at 18:56