The Young Monk asks Jim/"Father Brown" about his commentary on Revelation 10:19, a verse apparently about being kind to strangers. Not only does Revelation 10 have only eleven verses, but there is no verse in the book commanding kindness to strangers.
When Bob comes out from hiding on the shrine he fires a total of seven shots without reloading his six shot revolver.
When Ned agrees to partake in the march to Canada while talking to Father Levesque a boom can clearly be seen being raised in the reflection of windows behind them.