It was fraud, not an error
Some people are trying to minimize the impact of adding melamine to food by claiming that melamine is "only" as poisonous as table salt. No amount of melamine should be present in food; it is added for only one reason, and that is to fraudulently rig a standard test for protein.
Where does it end? A Chinese company sold ethylene glycol as "TD glycerine", a "glycerine substitute". Cough syrup containing the ethylene glycol "glycerine sunstitute" has killed several people, and some medical people estimate that the death toll may be in the tens of thousands.