big_mac_heart_attack posted this gorgeous example of a "fallstreak hole" with a rainbow in a cloud formation over eastern Victoria, Australia. They are rare enough that some people think they are evidence of UFOs. Unfortunately, that isn't usually the case. From Weather Underground:
Fallstreak holes form in these high to mid-level (cirrocumulus or altocumulus) clouds which are comprised of tiny water droplets that are below the freezing temperature but have not yet frozen (called supercooled water droplets). Airplanes passing through the cloud help the supercooled water droplets freeze. Air expands and cools as it passes over the wings and the propellor blades, decreasing the ambient temperature just enough to allow the droplets to freeze. The ice crystals grow and start to fall, while causing the water droplets around the ice crystals to evaporate. This leaves a large hole in the cloud with brush-like streaks of ice falling below it.