The Mars Climate Orbiter was a probe meant to orbit the planet Mars and study its atmosphere as well as act as a transmission relay for the Mars Polar Lander. It was lost in 1999 as it was trying to reach its orbit, because some of the computer programs directing its behaviour used non-SI units (pounds per second instead of Newtons per second). This caused the trajectory of the probe to be inadequate for orbital insertion, and the orbiter disintegrated in the martian atmosphere.
A silly mistake that cost over 300 million dollars. Still, as far as accurately sending projectiles go, an impressive feat.