The Great Plot Holio *Possible Spoilers*
Toward the beginning we see the father and son aliens collecting the last of the "fuel". The tiny drop they put in made the difference between having sufficient fuel to leave, or not. Then Wikus gets sprayed in the face with much more than that amount. How did they still have enough "fuel" in the canister? You could also assume the corporation took some out for testing as well, though that wasn't shown.
Was this addressed and I missed it, or maybe edited out but in the script?