So it turns out that Macbooks can slowly electrocute you. Those pretty Aluminium unibody machines have a pretty serious fault in certain cases. This can also cause problems with peripherals, since the various plugs come into contact with the body of the machine.
So to make this quick, the problem is that if you don’t use the three-pronged adapter when you plug your Macbook into a power outlet (you use the tiny travel-adapter instead of the long cord attachment, for example) you may feel a tingling sensation in your palms as you use the computer. If you’re not sure, try dragging your palms across the palm rest with your feet planted firmly on the floor. If you feel a weird “texture” (for lack of a better word), that might be electricity. You can confirm something is going on by unplugging your power adapter and trying again. If the feeling is gone, then you’ve been acting as a ground for the Macbook.
I first noticed this sensation shortly after I got my used Macbook 2013 15″ from Kijiji. It came with the travel (two-pronged) adapter. I replaced the two-pronged attachment with a proper three-pronged one (with the ground, that’s what that third, round piece of metal is), and the feeling is completely gone. The same thing happens on my backup-Macbook, a 2010 15″ if I switch the connectors.
My external burner didn’t work with my new Macbook either, I thought it was a software problem until I tried with the grounded adapter, and it suddenly worked great again, so the improper grounding affects peripherals as well, and could potentially damage hard drives, phones, and anything else you could plug in to the computer.
If you have a good look at the MagSafe power adapter, you can guess that the entire metal magnetic part which surrounds the pins is probably the ground, and it touches the computer directly, so you can safely assume that Apple intends the entire body of the computer to be grounded via the MagSafe adapter. If you happen to be a better ground than the adapter, then the electricity travels right through you into the floor. This is a pretty big design flaw, at least if you use the non-grounded adapter. So don’t do it! It’s probably a good idea to spread the word, too, since Apple don’t seem to know anything about this, or believe people who notice the problem, but a quick Google search will reveal threads like this one: https://discussions.apple.com/thread/3969131?tstart=0 advising you to wear rubber slippers to avoid the shocks, which is really too much to ask of even a die-hard mac-lover.