A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.
The last time anybody made a list of the top hundred character attributes of New Yorkers, common sense snuck in at number 79.
A computer terminal is not some clunky old television with a typewriter in front of it. It is an interface where the mind and body can connect with the universe and move bits of it about.
The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at and repair.