Apollo 6
Because of the launch problems, the rocket did not reach the planned height. After two orbits, the third stage, S-IVB would not restart. It was decided to use the Apollo Service Module engines to get the spacecraft into a higher orbit, as had been done in Apollo 4. It burned for 442 seconds (longer than it would ever fire on a lunar mission) to get to the planned  height. There was not enough fuel to speed up the atmospheric reentry. The spacecraft only entered the atmosphere at a speed of  instead of the planned  of a lunar return. However, this had been demonstrated on Apollo 4.