Looks like my Garmin lost signal at least once. In the early miles of the race it would peep mile markers before I hit the official ones on the course, but by the end it was peeping just after I passed mile signs. This isn't too surprising...I had the same thing happen doing the North Country Trail Relay. Garmin units don't do particularly well under dense tree cover...particularly if there are any noticeable hills around. There was a fair amount of both on much of the HM course. It was right around the 5-6 mile point that my Garmin went from alerting me of miles prior to the mile markers to alerting me on or after the miles...and that big drop on the elevation profile would suggest that signal was lost for a little bit there. Forerunners like to cut corners, too.
Man, it sure was nice to have it to help me hit my pace, though. I knew I wanted to run this conservatively, but still PR...and I did. My Forerunner is a great reality check. One of these days I need to actually try it with the heart-rate monitor strap...especially since I have had it for almost 2 years and have never once done so.