I do have this problem. I think it is a design error from Itead. Some "noise" in the wiring caused when switching other lights makes Sonoff Mini to detect voltage variation in S1-S2 pins. I hope they solve this, I bought 20 Sonoff mini.
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11 months ago
Now that you mention it you are right when I turn on the mini a very low sound is heard as if it were some frequency but it is heard. It works well with the app but not with the physical switch.
10 months ago
I have the same problem. I had installed the sonoff mini just two weeks back to control a door lock both via manual switch or via sonoff mini and the mini has erratically turned on 3 times already in the first two weeks so I had to remove it.
This is so unreliable and surely needs a fix