• So, I suspect the original problem was that the input signal was floating.

    1. If you connect a button only to GND (or VCC) you need a pullup (or
      pulldown) resistor. (done in #10 with an internal resistor)
    2. If you connect a button to GND (or VCC) when not pressed and to VCC (or GND) when pressed there is no problem.

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