I was testing a rocket which had 8 astronauts seated in a cargo bay, with one more in the command pod, no engines yet. I got the main astronaut out of the command pod and tried to catch the rover that was next to the rocket. I clicked on the seat to enter it, which placed the astronaut in the seat. It did not actually place him in the seat or give him the 'sitting' animation, though. I noticed the command pod's menu panel said the seat was empty, so I clicked the enter button again. I got the message that the command pod was full. So, I closed the menu and reopened it again, which showed that there was actually a crew member in the chair. I EVA'ed him, which resulted in his animation sizing up and him rolling like a bowling pin all over Luna. So, I grab one of the astronauts out of the cargo Bay and tether him to the ground. I wait until he stops moving, then go after the rover. Once I catch up ti the rover and tether it, I attempt to enter the chair again. Same problem.
I think this may be related to either Luna's gravity or the number of astronauts in the world. (10-11 astronauts)