I think I've figured out why ships "overshoot."
Acceleration, as we all know, is not only "increasing velocity" but actually "change in velocity." If you set a super-long distance route to try and exploit the acceleration (like we have all been experimenting with), your ship will not be able to decelerate to "hit" the planet. Thus, you will overshoot the planet. This could happen, I guess, any time you change course at all. I think you'd still fight any ships in your path, but probably not be able to land on a planet.
I have 2 arcs that overshot without me changing the target as far as I can remember. So it may also be !@#$ty piloting. :-)
I have a hunch that it does not calculate fractional acc/dec - so every turn, it checks if it should accelerate or decelerate, but it doesn't take into account that the halfway point isn't precisely on the turn.
