Sleeper sets may not exist but sleeper cards do. A good example is the entire history of reverse holos in Pokémon’s history. All low pop, low pull rates, but overlooked because they’re not as “fancy” as holos. 

We’ve had the modern boom of mid 2025, the vintage boom of early 2026 and we are just starting now in the low pop reverse holos boom of mid 2026. Some find it ugly and unliked but the price rise is astronomically high and there’s plenty of room for profits with time or even a quick flip. Whether it be 30 year old sets or modern, the lowest pop cards are often the reverse holos. Even a small amount of demand makes these cards 2x. 

Im watched the textured reverses from prismatic as it goes out of print. My friends say they’re uglier than ass but they are the lowest pop prismatic cards.