I did a quick look at this, and it seems like the trigger conditions are:

1) Plan a move that goes (at least) three turns into the future (so the turn indicator for the move is a "4").

2) Have one of stages (turns) of the move end on a friendly unit.

If the turn indicator is less than 4, the planned move is trashed instead of this assertion failing. (When the turn indicator is 4, the C++ variable turn_num is 3, which no longer satisfies the second half of the assertion.)

This does not occur in 1.11. (In 1.11, the same conditions cause a "dashed arrow" to indicate the problematic move.)

When I played this scenario the first time and other reloads, I have this same error.

"Assertion failed!
src\whiteboard\side_actions.cpp, line 631
Expression: turn_num <= actions_.size() || turn_num <= 2"

>systeminfo | findstr /B /C:"OS Name" /C:"OS Version"

OS Name: Microsoft Windows 7 Home Premium
OS Version: 6.1.7601 Service Pack 1 Build 7601

It happens when I move the 'Walking Corpse' from hex location (23,11) to (10,18) by 'planning mode'. See attached picture "bug0".

I get this message (is it an exception? I am not sure)
"bug1"
It is taken screen-shotted next to the line of the code I believe where the assertion is thrown. Even though I downloaded the code to look at, this error was thrown from the 1.10.5 Win executable downloaded - It was NOT ran from my own compiled attempts.

I have also attached auto saved files, you need to start from #4, as that is where the zombie initially is.

Notes:
I am trying to enque the unit's path to hit all 4 of my allies villages, started with the closest one. I notice that if I enque any of the other 3 villages to the far west this error does not happen.

Recreating the Bug:
1.) Load "DiD-Saving_Parthyn-Auto-Save4" and try to move the 'Walking Corpse' unit from (23,11) to (10,18)

Another way to recreate the Bug:
2.) Load "DiD-Saving_Parthyn-Auto-Save4"
IF you hit "end turn" without performing ANY action, this error will happen again ONLY if you attempt to move the 'Walking Corpse' unit from (23,11) to any of the far two villages (2,13) and (6,14)

-- However, if DURING turn 4 you move your units around the river with a mix of normal moves and planning mode (some of them I moved immediately via normal moves and planning mode, one unit I enqued for two moves) you will NOT get this error when you attempt to move the 'Walking Corpse' unit on (23,11) to any of the four far west villages. This replay file has been included.