Traffic Jam
Many members of the American Society of Association Executives, attending the organization's annual meeting in Chicago, commented (but really didn't complain) about delays getting from the hotel to the convention center because of the throngs of runners striding through downtown Chicago on Sunday morning.
The 8,719 participants in the LaSalle Bank Chicago Distance Classic Half-Marathon endured early morning rain and muggy weather as they traversed the 13.1 mile course, not aware of (and, probably, not concerned with) the travel needs of the meeting goers.
When scheduling your race, always make sure there is no conflict with other events - conventions, fairs, street construction, etc. Fortunately, municipal officials often feel that more than one activity can be accomodated on city streets, so you may not have a problem. But, it is always best to be sure that everyone involved knows what else is going on that day so nobody is surprised if a problem occurs.