Judges 20:39
And when the men of Israel retired in the battle, Benjamin began to smite and kill of the men of Israel about thirty persons: for they said, Surely they are smitten down before us, as in the first battle.
Cross-reference
Judges 20:31
Parallel
Judges 20:31 records the same 'about thirty men' killed earlier — here the Benjamites mistakenly think they are repeating that success.