This question keeps popping up and several people believe to know of a situation where 2 rh negatives have had an rh positive child. The answer:
It is not possible.
If you are truly rh negative, you cannot carry the D recessively. You can however be a weak-D. Meaning you are rh positive, but the rh(D) is so weak, sometimes you might test as an rh negative and sometimes as an rh positive.
See also:
4 major reasons why blood type tests can show different results