South Africa 2007 (98,552), United States 2000 (4,727) and Puerto Rico (8) have zero weigthing
For South Africa, all respondents with a zero weighting are those that were not living in a dwelling within the Census scope (e.g. prison, boarding school, etc.). These respondents can be identified with the variable ZA07A_DWTYPEP.
For the United States and Puerto Rico, the Census Bureau generated these weights to represent the 100-percent totals for certain data groups. A weight of zero simply indicates that the individual was “over-represented” in the sample.
Refer to this previous answer about Brazil for information on another case of zero weighting.
Hope this helps.