It could be that the USB wall adaptor was particularly 'noisy'... I guess cheap ones are probably designed to output a lot of power (1-2A) at 5v, and when Espruino a plugged in and draws 10mA, the signal is probably pretty noisy.
The other thing you could do is to call E.getAnalogVRef() - this tells you what voltage Espruino thinks it is running off. It should be ~3.3v, but maybe the wall supply goes into some kind of power saving mode and the output voltage drops below what Espruino needs?
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