Why is mains voltage showing up on my oscilloscope from MagSafe shield?
I guess the USB shield isn't grounded. Strange but not impossible.

Without a ground reference, the scope will measure the difference between its own ground or chassis, and the laptop ground. If the laptop has only a two pin plug, then its ground will be floating at about VSupply/2 (RMS) because of the capacitors in its EMI filter.

Clip the scope ground to the Arduino ground to force the laptop ground to the same as mains ground, and the scope will start to read more normally. A small current will flow, it's limited by law to perhaps 200 uA, but might be a lot less.

In general on an experimental bench, you should connect the equipment ground to mains earth. The 100 V at 100 uA won't hurt you, and your laptop will be protected against it, but the Arduino isn't, and you can easily blow things by touching them.