At one of my recent favorite regional trail systems, there is a ~1/2 mile gravel trail that leads to the main hardpack singletrack. The whole system is well maintained, so the gravel is new and fairly loose and a nice vivid gray.
Well the wife and I were sorta racing to the trail on this gravel trail, with mtbs and knobby tires we were struggling to maintain ~16mph. Then we both saw a little purple dot sitting in the middle of the trail. It looked like a toy that a kid had dropped. As we quickly approached it, I noticed it wasn't as far away as it seemed, it was a lot closer, because it wasn't on the ground, it was about 2 ft off the ground. So I started wondering how a purple thing would be floating in the middle of the trail, a big bug? a purple hummingbird? Then i saw 2 posts to either side of the trail, and immediately shouted "SSSSTTTOOOPPP!!!!" we both slammed on the brakes and skidded madly up to about 2 feet from this purple thing, and we saw it was a dot of purple paint someone had sprayed on a braided steel cable, probably to mark it as it was the same color and shade as the gravel.
We didn't have much fun on the rest of the ride as we were paranoid about more of these gates. Perhaps a chicken wire gate that had rusted to match the color of the reddish dirt.