Did they?? I seem to remember them getting it right... it's getting 4 litres from a 3 litre and 5 litre bottle. Fill the 3 litre, poor it into the 5, fill the 3 litre again, poor as much as you can into the 5 litre bottle (space for 2 litres, leaving 1 litre remaning in the 3 litre bottle). Poor the 5 litre bottle out and put the 1 litre that's in the 3 litre bottle into the 5 litre bottle, refill the 3 litre bottle, poor all of that into the 5 litre bottle and you have 4 litres.
That's what they did in the film right?