The mod Multiple Unit Train Control (1.0.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event MultipleUnitTrainControl::on_tick (ID 0)
__MultipleUnitTrainControl__/control.lua:820: 

[ER Global Lock] Forbidden global *write*:
{key = "inventory", value = 1}

stack traceback:
    [C]: in function 'error'
    __MultipleUnitTrainControl__/control.lua:820: in function '__newindex'
    __Robot256Lib__/script/save_restore.lua:475: in function 'saveItemRequestProxy'
    __Robot256Lib__/script/carriage_replacement.lua:75: in function 'replaceCarriage'
    __MultipleUnitTrainControl__/control.lua:164: in function 'ProcessReplacement'
    __MultipleUnitTrainControl__/control.lua:275: in function 'ProcessTrain'
    __MultipleUnitTrainControl__/control.lua:377: in function 'ProcessTrainQueue'
    __MultipleUnitTrainControl__/control.lua:405: in function <__MultipleUnitTrainControl__/control.lua:402>

What version of Robot256Lib are you using? Please update to the latest version.

2.0.3

Can you post factorio-current.log showing the crash? (I'm confused because line 475 is empty)

I moved on, but I will do so if/when it happens again

I seriously don't know why the "please report to mod author" message doesn't say "please send log file to mod author.