Sandbox warning

"Danger. Deadly beams ahead."

The Sandbox Warning Easter Egg can be seen using the Out of Sky Bubble Glitch on Sandbox.

Tutorial
Using the glitch you just get close to the Guardian Tower left of the setting sun. (Not too close because there seems to be a kill wall preventing you from touching it.)

As you get closer you should eventually see an altered version of the Splatter medal. The difference is the Gold cross has been removed, the many-pointer star background has been replaced by an explosion and the tube-like guts has been changed to what appears to be a zipper. Apparently, according to some people, this egg tells you what the tower does.

Trivia

 * The kill wall only seems to be on the tower on the left of the sun. Others seem to let you stand on them.
 * Through overloading the map and looking through a sniper rifle, it can be seen but only faintly. In order to see it properly, you must perform the Out of Sky Bubble Glitch.