Albert Barnes Commentary Ecclesiastes 10:11

"If the serpent bite before it is charmed, then is there no advantage in the charmer." — Ecclesiastes 10:11 (ASV)

Rather: “If a serpent without enchantment (that is, not being enchanted) bites, then there is no advantage to the charmer”: that is, if the charmer is unwisely slack in exercising his craft, he will be bitten like other people. See Psalms 58:4 note.