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In the early days of computer terminals, it was not uncommon to have keyboards that featured no more than "2-key rollover". That meant that if you hit more than 2 keys closely together -- common with touch typing -- the results would be, well, not what you wanted.

This was, however, better than the situation with IBM keypunches, which had mechanical keyboards with 1-key rollover. That is, you could only type one character at a time, and all other keys were physically locked out until you released the first key.

Fun, eh?