Not an urban legend, it really can happen. This was tested and proven back in the 1950s or so by some gun rag writer. Under recoil the rifle moves backwards, but the ammo would travel forward in the magazine tube because of inertia. When the ammo is forced to the rear by the spring and hits the cartridge stop, the force can make the tip of the bullet detonate the round ahead of it if it strikes in the center of the primer. Now, what are the chances it will happen at any time I don't know, but yes, it can happen. But since there are no tube fed-lever actions in .243 made commercially, it's not really a concern.