Unsure but this has happened in two tests when using a Headless Client and assigning ownership of objects by the server.
CODE being used:
_hcID = owner (HEADLESS_CLIENT_ENTITY);
_list = allUnits+vehicles;
{
if(owner _x IN [0,1]) then {
if( _x setOwner _hcID ) then {
if(debug_log) then { diag_log format["HLC: %1 setOwner %2 - Done ",_x,_hcID] };
}else{
if(debug_log) then { diag_log format["HLC: %1 setOwner %2 - Fail -> Owner is %3",_x,_hcID,owner _x] };
};
};
}forEach _list;