Plot hole: Every single one of the ronin are portrayed as being extremely lax with their most prized possession, their swords. When they first gather, one of the ronin tells Oishi "We do not have any swords", whereupon Oishi casually hand over his own. First of all, only the most poverty-stricken and desperate ronin sold their swords, as giving up one's sword was the ultimate humiliation for a samurai, it was a denial of both one's status and legacy. Secondly, selling your sword would mean never being able to be employed as samurai again, or even to be hired as a temporary bodyguard. Thirdly, swords were often inherited from one's father, or given as personal gifts by parents or lords, not something one would willingly give up. It is possible a few of the ronin would have been driven to selling or otherwise losing their swords, but not every single one of the 50+ ronin who gather. And those who still had them would be extremely loathe to even lend them to others.
Continuity mistake: In the scene where the 47 ronin are granted a proper samurai's death, the shogun first has the black strap of his headpiece under his lip, then under his chin and then under his lip again. (01:41:00 - 01:42:40)