In 1991, amongst a series of theorems about rounding in floating point arithmetic calculations, the author parenthetically noted that rounding a number to 53 significant digits, and then rounding it again to 52 significant digits, might produce a different answer compared to when you do the rounding to 52 digits all in one go. He all but apologised for the parenthesis, noting it was "highly unlikely to affect any practical program adversely."
19 years and 10 months later a researcher discovered that the "Mark of the Beast Bug" could freeze almost any computer in the world, and that millions of webservers could be taken down by it almost at the touch of a button — because of an error when rounding a very very small number, first to 53 significant digits, and then to 52.