Quote from rayl:
Crossed quotes are actually not infrequent. I've seen at least the following issues:
1. problems w/some exchanges (e.g., I know I saw a DirectEdge issue earlier today) with bad quotes
2. Time lags in updating quotes from some exchanges.
3. Less efficient markets where there isn't NMS trade through protection, e.g. OTCBB montage crossing ArcaEdge quotes. (And actually NMS isn't fully phased in yet.)