They are very similar to average-looking Krabby Patties from the exterior, but upon biting into one, they are clearly spray-painted with a gray goop substance from a patty-making machine.
Role in episode
They were made from a gray-like substance that comes out of a machine, heated and cooked in a furnace, and finally spray-painted to look like a regular Krabby Patty. Seeing the process sickens SpongeBob and Mr. Krabs and eventually, the latter reveals the inside of the patty to all of Krabby O'Monday's customers. At first, the customers enjoy the patties, but as soon as they see what is inside, most of them flee the restaurant and barf it out.
People would notice the flavor of the patty changed dramatically, especially if they were regular patrons to the Krusty Krab, and since the gray substance was used for all of the components of the Krabby Patty (bun, patty, toppings, etc.)
The customers should've noticed that it was gray inside, since people usually look at their food before consuming it.
The gray goop used to make one could be a reference to the pink slime that was falsely accused to be used in McDonald's food.