From the NYT Magazine of the 18th, in a “Who’s Who” section on “Obama’s People”, a brief identification of Nancy Pelosi (p. 110):
(1) NANCY PELOSI, 68, grew up in Baltimore, where her father and brother were mayor.
This struck me as a bit off, because of the plural subject (her father and brother) and plural verb (were), but singular predicative (mayor). Normally, these three constituents agree in number (though there are constructions where non-agreement is possible, as in “The problem is rats”):
Her father and brother were politicians/*(a) politician.
Shifting the predicative to plural in (1) is possible, but (to my ear) still not entirely satisfactory:
(2) NANCY PELOSI, 68, grew up in Baltimore, where her father and brother were mayors.
What’s going on here is that mayor in (1) is a singular count noun, used (exceptionally) without a determiner, to denote a “unique role” (which is why the shift to the plural in (2) alters the sense of the example). Nevertheless, (1) takes a little work to understand, since the reader has to work it out that these two people filled this unique role at different times.