The transfer from pen and paper methods to hand calculator methods which has been made by many teachers still uses visible denary digits. We are now beginning the transfer to a computer and I feel that the Mathematical Association’s admirable new report does not place enough emphasis on the possibilities of computer system errors which are invisible to the user. The teacher looking at numerical error must not put untested faith in its abilities, and must not expect it always to behave in a regular denary fashion, as will be shown in the tests below. He must also be warned that the computer reference manual rarely has the reliability of a text-book.