The Number Of...

In the two occurrences above of the number of cases (Straits Times, 10 December 2009), the writer uses a singular verb is in the first instance (correct) but plural go in the second (wrong).

The rule to remember is this: use a singular verb with the number of ..., but a plural verb with a number of ....

This is because, when we say the number of ..., we are really referring to a number.  Since number is a singular head noun, it accordingly takes a singular verb.

But when we say a number of ..., we actually mean some.  Hence, a number of becomes a complex determiner.  Since the noun that follows of is always plural (e.g. a number of doctors/cases, not a number of *doctor/*case), the verb should also be plural.