EVA Construction Bug Creates Zombie Kerbals and Kills SAS
Win 64, stock KSP 1.11.2, All DLC, updated over time.
So I've had a bug occur several times where I board a ship with an engineer after doing EVA Construction work, only to still see that Kerbal on the ladder while simultaneously seeing him listed as being in the ship. The problem is that the Kerbal is listed as alive inside the ship as he should be, but a glitchy zombie Kerbal appears from another dimension or something and stays stuck on the ladder.
This causes a whole host of problems.
1: I can switch to that zombie Kerbal on the ladder, only to not be able to see anything around, and I have to hit esc and go to the tracking station to get away, or if I can see the area, I can't control him. I also cannot see his portrait in the corner, but a bugged black screen with some code error messages in the name field.
2: This automatically kills the SAS unit of the ship the Kerbal boards, making it extremely hard to hold stability or dock anything at weird angles as long as the zombie Kerbal is alive. I think it sorts itself out if you explode the zombie during re-entry, but don't quote me on that.
3: This zombie Kerbal sometimes prevents other Kerbals from exiting due to a blocked hatch, and other times acts more like a ghost, allowing Kerbals to exit and exist in the same space as the zombie Kerbal. It is unpredictable in this manner, and others.
3: This zombie Kerbal is still considered alive somehow, and still considered to be on the ladder, and often shows up in the map/tracking station, even though he is technically also a separate Kerbal of the same name alive inside the ship as well.
4: Time warp sometimes fails to function due to a zombie Kerbal being on a ladder.
5: During re-entry, this zombie Kerbal often overheats and explodes and can be seen in the F3 screen as having died, and may appear in the astronaut complex as being KIA, though upon landing and recovery, the Kerbal is still alive and listed in the roster, as his real self was recovered alive inside the ship.
So basically, it seems the zombie Kerbals are not all created equally, but they are all created by the same bug, and they're really annoying.
As for reproducing this, just go to a space station and do lots of little mods to it while drifting nearby in various ships and re-board your ship several times. Maybe try boarding while holding parts or while still in construction mode to trigger it more often. I haven't pinned down the exact trigger, but it happens about 5% of the time when I do EVA construction now. I almost gave up on EVA construction in the beginning because I had this problems several times immediately upon trying it, but it has been less of a problem for me lately, though still definitely there.