It costs a boatload of money to send stuff to Mars. Not just the launch and the fuel but the development and production is expensive too. That is why NASA contracted with the European Space Agency to develop the propulsion section of the Mars Climate Orbiter (MCO).
In order to place itself in proper orbit around Mars, the MCO was going to have to “tap the brakes” to slow enough to allow the gravity of mars to put it into orbit.
In Europe they have a different way of measuring things than here. They use metric, we use the Imperial system. In the imperial system a rocket motors thrust is measured in pound force. The unit of measure in the metric system is the Newton.
It was later determined that this difference in systems of measure lead to a problem. The software controlling the propulsion system had never been thoroughly checked and a computer underestimated the effect of the thrusters by a factor of 4.45, the ratio of newtons to lbs force.
We didn’t “tap the brakes”. And the orbiter made a new crater.