If you have a maneuver node placed while orbiting the sun, then your craft enters the SOI of a planetary body, the maneuver node disappears and you are unable to create another node.
For example - I was orbiting the sun on an approach with the Uranus analog planet. My approach vector showing the encounter disappeared after warping close to the encounter, so I placed down a maneuver node to try to quickly adjust course. While adjusting the node, my craft passed into the (invisible to me) approach vector, causing my maneuver node on the flight path orbiting the sun to disappear, and me unable to make a new maneuver node on my approach vector flight path. Had to save, exit, and reload to clear the bug.
This may happen under similar circumstances with other SOI change combinations while maneuver nodes are placed.