i was visiting the mun arch when my craft fell over, so i sent a rescue mission but when i got there and switched back to the space craft the craft fell through the muns surface and was in freefall to the core of the mun, it seems to only happen when i switch crafts at a distance of around 1km or less. after passing the core of the mun it got a super powerful gravity assist and sent the space craft back to the surface at around 2000m/s, the craft then passed through the terrain and left the muns sphere of influence.