Dewford Gym footprint glitch
The Footprint Glitch or sometimes specifically the Dewford Gym Footprint Glitch is a minor glitch which only exists in Pokémon Ruby and Sapphire. When the player walks across sand he or she will normally leave footprints, however if an obstacle such as the Dewford Gym is one layer above the player no footprints will be left even if sand is underneath the player's feet.
External links
[1] - A list of minor glitches in Pokémon Ruby and Sapphire from Pokéglitch.tripod.com; one of Glitch City Laboratories' ancestors.