What is the mistake spoken of in this quote from "Arms and the Man"?"...and there shall be no mistake about cartridges this time".Why are cartridges needed once again?
This quote is spoken by Bluntschli to Sergius in Act III. Bluntschli is referring back to a conversation he had had with Raina in Act I, when he was a fugitive soldier hiding out in her room. Bluntschli, who had been fighting for the Servians at that time, had just escaped from a battle in which his side had been beaten. He explained to Raina that the Servians should have won that confrontation, except for the fact that they had been sent the wrong machine gun cartridges, and therefore had been prevented from firing a shot while the enemy Russian/Bulgarian cavalry charged them. The leader of the cavalry charge had been Sergius, who had foolishly let his men directly into a Servian stronghold which should have decimated them, but because of the mistake with the cartridges, the charge had been successful and the Servians had been overcome.
In Act III, Sergius has just challenged Bluntschli to a duel, so the two men will conceivably face each other in battle once again. Sergius is engaged to Raina, but has reason to believe that Bluntschli is a rival to his betrothed's affections. Bluntschli responds to the challenge by saying that his choice of weapons for the confrontation is a machine gun, and he warns that should they duel, Bluntschli will have the proper ammunition for his gun this time - there will not be another mistake about the cartridges, and Sergius should not expect an undeserved victory a second time.