Speculation has been running rampant that the Rock 'n' Roll Denver Marathon course was long, from people reporting mileage of 26.7 on their Garmin, to investigative runners checking out the USATF map and their Garmin uploads.
The error appears to have occured on 8th Avenue. If you look at the Garmin map above the runners turned around at Tejon Street. If you look at the USATF map you'll noticed that runners turn around before Tejon Street, at the east side of the building. This was around mile 24 on the course.
Not exactly sure how much this lengthened the course, but it appears that the course was long. Maybe a 1/4 of a mile, just a guess.