I was riding my old MTB (I think it must have been 3-4 months ago) and I was practicing riding without touching the handlebars. I veered off into the dirt off to the side of the Highline Canal trail in Denver (cyccommute might know where this is; near the golf course, where the highline canal meets the cherry creek trail) for about 10-15 feet, and I must've had 10-15 goatheads in each tire. I noticed them on my front tire as it was spinning, so I pulled over and checked to make sure they were really goatheads. Indeed they were. This was before I started using SLiME (works great) in my tubes and started carrying a patch kit. I rode for about 3 miles after I noticed them, and then the tires got too flat to ride, so I called me mom and she picked me up. She was at an appointment, so I just walked my bike until she could pick me up (about 3 miles, and I was half a mile from home).