The most famous smallest but most significant human error was Mariner I:

"The error had occurred when a symbol was being transcribed by hand in the specification for the guidance program. The writer missed the superscript bar (or overline) in [the formula] by which was meant "the nth smoothed value of the time derivative of a radius R". Without the smoothing function indicated by the bar, the program treated normal minor variations of velocity as if they were serious, causing spurious corrections that sent the rocket off course. It was then destroyed by the Range Safety Officer."

http://en.wikipedia.org/wiki/Mariner_1