if there is a line that allows hauling of different types of cargo that go into the same vehicle (eg oil, wood, ore, coal, goods all go on a truck) and several types are available at a station, more loads are hauled than allowed. eg a truck with a capacity of 4, would then haul 4 wood and 1 ore instead of 3 wood and 1 ore or 4 wood.
it is quite easy to reproduce, as it happens very often if not always. it also happens with rail cars.
please patch this.
the build where this occured is 4221, win-32