Revealing mistake: When the 47 Ronin write down their names to swear fealty to the mission, they are cutting their thumbs on the swords to imprint their fingerprints on the document. The mistake is that they use the blunt edge of the sword to do this.
Revealing mistake: After Paul shoots the monkey he picks it up to put it in a box and the "dead monkey" is fake. (00:51:56)