The electric feeling is actually caused by EMI reduction measures: per regulations, a Y2 class capacitor must be connected between live and earth in your laptop's power supply. If you connect your charger to an earthed outlet (like you should), the earth is going to absorb the resulting high-impedance supply. If not, the resulting voltage will be present on your computer's metal chassis, which must be connected to the earth per other regulations on electrical safety. Of course, since the capacitor has a high impedance (reactance, to be precise), no meaningful current can go through it, so the "seemingly mains-voltage" on your chassis isn't dangerous at all (as long as everything is working as intended)

The reason for that is that the adapter is not really grounded, but due to EMC requirements there has to be some AC path to ground. This is done by connecting the output ground to midpoint of input EMC filter and thus the output ground is weakly capacitively coupled to half of the line voltage.

Most higher powered laptop power bricks have three prong plug for exactly this reason.

Yeah I'm using a Macbook Pro power adaptor and an Apple Cinema display. Both have earth pins (UK plug). Somehow their 'grounds' disagree enough to ionize air!?

Are you plugged into the same outlet? Different outlets could be on different circuits which can be enough to cause a ground difference.

Yes, the plugs are next to each other. And a mains tester is happy with the mains output.

Oh! I know about this! It's the "fuzzy" feeling you get when you touch a MBP when it's plugged in!

It only happens when you're using the two-prong "duck bill" on the power adapter itself. If you use a power cord with a three-prong plug instead of the two-prong duck bill, the feeling isn't present.

This is a known issue with ungrounded electronics in general.

Apple Macbooks are particularly bad because the enclosure is metal and the standard plug (the interchangeable part of the charging brick) that is sold in most parts of the world is not grounded.

In my country (in Europe) a Macbook charger is sold with a 'type C', (AKA EuroPlug) which is not grounded. The extended cord that comes with the charger is a 'type F' plug, which is grounded. [0]

For me, this results in always having to lug around the extended cord with the charger, as the non-grounded variant makes my Macbook unpleasant to use.

[0] https://www.worldstandards.eu/electricity/plugs-and-sockets/

In the end, you can always buy a new macbook and that's a plus ;-)

I know this feeling, noticed it since at least the aluminium PowerBook G4 I had once.