Perhaps they taped a sheet of acetate over the screen.

That's the smartest part of the hack I guess - that the waiter actually invested time to test out his idea! Who knows probably the first marker he picked up didn't work the way he expected and he moved on to the next one till he found what worked.

It's more likely he doesn't give a monkeys about degrading the LCD display and the first whiteboard pen he found worked fine.

:D ... guess you are right!

A random pen I used for exactly the same purpose didn't do any lasting damage to LCD, so I guess it's not that difficult to find them.

If the LCD screen is a glass, like on Apple hardware, there is no problem. :-)

I thought Apple screens had special coatings on the surface of the glass.

In Sweden, they're all TFTs these days. Even the non-touch ones.

My experience with generic "Don't put this chemical on that surface" issues has been that it usually takes quite a long time of heavy contact before there's a noticeable effect. I'm guessing an Italian restaurant with an identifiable "head waiter" can afford to replace a $200 monitor every few months.