They were made from a gray-like substance that comes out of a machine, then heated and cooked in a furnace, then finally spray-painted to look like a regular Krabby Patty. The synthetic patties sickened SpongeBob and Mr. Krabs and eventually, Mr. Krabs revealed the inside of the patty to all of the Krabby O'Monday's customers. At first, the customers enjoyed the patties, but as soon as they saw what was in the patty, most of them fled the restaurant and almost barfed 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 the components of the Krabby Patty (bun, patty, toppings, etc.)
The customers should've noticed it was gray inside, since people sometimes usually look at their food just before taking a bite out of it.
The gray goop used to make one could be a reference to pink slime that was used in McDonald's food.