When manoeuvre is almost complete the manoeuvre marker on the navball shoots off

[THX1138]

Why does the manoeuvre marker shoot off somewhere when a manoeuvre is almost complete? What's going on? Am I meant to follow it?

Edited December 18, 2014 by THX1138

[Taki117]

This means that you are approaching the dV expenditure for the maneuver, but your velocity vector is off, don't worry about following it, as doing so will change your orbit far more than cutting your engines. A good practice to avoid this is to throttle down your engines to minimum thrust (no more than 25%) about 15 m/s from node completion to correct any minor errors.

[TythosEternal]

Let's say you have to burn 100 m/s in a specific direction, so you point your craft in that direction and light your engines. Thing is, you're just human and therefore you're not pointed EXACTLY in the right direction. You're pretty close though, so you do your burn and, for the most part, you are able to realize the majority of that delta-v vector. What's left--though it's a very small part--is pointed in the direction of the DIFFERENCE between your objective delta-v vector and your realized delta-v vector, which--by definition (if the burn is of similar magnitude and very close to the objective)--will be very near normal to either (that is, almost a full ninety degrees away).

That's where that jump is coming from--by the time you've nearly completed your burn, all that's left is the error. As explained above, this error can come from small differences between your objective and actual burn vectors. It can also from numerical sources, or as a result of the non-instantaneous nature of your maneuver, which means your ship isn't pointed exactly where it was when the maneuver was planned or when it began, because as you've been burning your ship has still been flying around a curved conic section.

The solution to these jumps, short of backing out and controlling a perfect pointing and tracking solution for an integrated burn, is to simply ignore the blue nav ball pointer once you near the end of your burn. Instead, watch for the moment where the burnometer (the name I just made up for the delta-v counter to the right of your nav ball) turns from red to green. That's KSP's way of saying, "eh, close enough." You can then cancel the remaining maneuver node, switch to map mode, and make any minor corrections that are needed. You can also ignore the moving blue pointer entirely, staying prograde (or whatever direction your initial burn was in) and controlling your burn from the map view instead, where you have more direct feedback regarding your maneuver's success (are you on course for your target SOI, periapsis for aerobraking, etc.). This is what I do when shooting for aerobraking periapsis several SOIs away, because it's confusing, frustrating, and a lot of work to fine-tune as many maneuver nodes as you'd need to place to get that ain juuuuuuust right.