I have a curious occurrence that happened 3 times on 3 different interstates in the Seattle, WA area. I was humming along fine on the interstate, then for some reason Navigator (free) told me to get off on a slip road, then a few twists and turns, then back on the same interstate I was on. The first time, I thought that was my error, but after 2 more times, I don't think so.
Aside from that and another anomaly, I love it. It's going to retire my Garmin!
Yes its the OSM free maps. I had that a number of times in Kansas and Colorado. I have even seen that happen here in Germany. You can join the OSM community at the link Thomas has posted and be one of the thousands of people who improve the maps in their local area. Have fun and welcome to the club.
As already said, most probably this is due to missing maxspeed tags on the slip roads. You can check this here: http://www.itoworld.com/map/124 You could check the routing on the main page osm.org for errors.