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Not to nit pick, but shouldn't you have a resistor in series with the LED to limit the current in the output stage?
i do realize that this is 3 years old.but, my guess would be that since you are battery powered your finger has no real potensial in respect to the circuit. if connected to to usb charger or similar connectet to the mains, you have a potential in respect to ground. when you toutch the led pin this also applies
Breadboarding: The Darlington Pair
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