Whoso breaketh a hedge, a serpent shall bite him - While spoiling his neighbor's property, he himself may come to greater mischief: while pulling out the sticks, he may be bit by a serpent, who has his nest there. Some have supposed that נחש nachash here means a thorn; perhaps from the similarity of its prick to the serpent's sting. He who forces his way through a hedge will be pricked by the thorns.
The figures seem to be taken from the work of building up and pulling down houses. In their general application, they recommend the man who would act wisely to be cautious when taking any step in life which involves risk.
Ecclesiastes 10:8
Breaketh an hedge - Rather: “breaks through a wall.”
Serpent - The habit of snakes is to nestle in a chink of a wall, or among stones (compare Amos 5:19).
Ecclesiastes 10:9
Be endangered - Rather: “cut himself.”