Action DooM! Easter Egg - Massmouth Flakes!

When you start the City Level, go as normal until you reach the point where you have to climb over the car to get past the rubble. look to your right and you should see the bottom of a billboard. Take a few steps back, and you will be able to see that the board reads:

"Ask your grocer about NEW Massmouth Flakes!"

Of course, this is a reference to the insanely popular doom 2 wad "Massmouth" and it's sequel, "Massmouth 2" By Cyb.

The first Massmouth game features you, Massmouth, an alien with one eye on a stalk and a large mouth (Based on an old Quake skin) where you must hunt for rare items for your boss.

People speculate that this egg was probably left by Cyb himself, as he did do alot of work on Action DooM and its scripting.