Unexpected white feathers is a common phenomenon among birds apparently. A post at B and B describes A freaky chipping sparrow. And Troutgrrrl at Science and Sarcasm wrote about a white-tailed mourning dove visiting her yard. I myself have seen a house sparrow with patches of white on the wings where none should be.
Something else to take into account when identifying birds. Mike of 10,000 Birds has a series, Bergin's Laws of Avian Identification that is well worth taking a look at. His law number #4, "Common birds are easier to identify than rare birds," is pertinent here. Know your common birds well, don't take them for granted, and you'll have a better chance of not getting tripped up by one with an odd patch of white or two. (B and B's freaky chipping sparrow was much whiter than that--that was a true feat of identification!)
I read something about this white feather business just in the last week or so, suggesting that there are a number of possible causes. One is diet, and in those cases the phenomenon might well disappear at the next molt. Unfortunately I can't remember where I read it--but if it comes back to me I'll add the reference.