I disagree. Statistically, it is highly improbable that, taken as an aggregate, that there wouldn’t be at least one vote (error or otherwise) for Romney.
100% is extremely unlikely, even with a homogenous demographic.
However, all things (political parties) are not equal. The Republican brand is so extraordinarily toxic in inner cities that such a random occurence would in fact be close to impossible. There may even be some mechanism to prevent or avoid mistaken Republican votes (which would send off red flags everywhere) in such precincts.
I think we are fighting a dire situation - for a large segment of the population (around 40%), voting Republican is like committing suicide (or so they believe).