DC voltage measured on an AC to AC wall wart - why and how?
Assuming this was a digital voltmeter, DMMs take samples of the input, if the sample falls on the same point on the AC wave form each time a measurement is taken, it'll (incorrectly) report a DC value. E.g. if the meter only sampled at the peak of an 8V peak sine wave signal, it'd report that it was seeing 8V DC, it doesn't see what's happening between samples so it's unaware it's looking at an AC waveform. This is a special case of something called "Aliasing" and can result in all kinds of nonsensical results if it's not properly accounted for. If you've got another meter (ideally a different model) see if it gives the same readings (you'll either get zero, a different DC value or a slowly changing value depending on whether the sample rate is a multiple of the AC frequency or not)