Because there is no level 27 yet. The bonus levels are there as a little incentive with free donuts until they add more levels.
Unless you get harpied.
It's basically a glitch that locks you out of the game and has a repeating harp sound on the title screen.
Nobody really knows what causes it. Big, nasty glitch. I'm lucky as I've never had it. But I play on an iPhone 5. Don't know if that helps.
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