From “Beating a retreat” in The Economist, 9/24/11, p. 99 (on-line here):
… soot particles absorb sunlight, and so warm up the atmosphere. Then, when snow or rain wash them onto an ice floe, they darken its surface and thus cause it to melt faster.
This is 3sg or 3sg (snow or rain) functioning as 3pl for the purposes of subject-verb agreement (wash rather than washes), though a general principle —
(1) When all parts of a subject joined by or or nor are singular, the verb is singular; when all parts are plural, the verb is plural (Little, Brown Handbook, quoted in “Agreement with disjunctive subjects”, here)
would predict 3sg agreement (and I would have used 3sg in this case).
Intuitively, this is a kind of “notional agreement”, snow or rain being understood as ‘snow and rain, whichever happen(s) to occur’. This is an unusually simple example; in the other sg or sg = pl cases I’ve collected, other things are going on.